Why You Should Care About Software Defined Radio

It hasn’t become a household term yet, but Software-Defined Radio (SDR) is a major player on the developing technology front. Whether you’re building products for mass consumption, or just playing around for fun, SDR is worth knowing something about and I’ll prove it to you.

SDR Boils Down a Hard Problem

First off let’s reconcile what is meant by “radio”. If it sends or receives via radio frequency it has a radio in it. This means your WiFi router, your cellphone, your laptop, many water and electrical meters, your garage door opener (but not your TV remote, that uses light), wireless security system sensors, police radios, your wireless mouse/keyboard, and that quadcopter you keep crashing in the neighbor’s yard all have one. Radios are so prolific we’re tempted to tell you they’re in absolutely everything.

Ettus Research USRP N210


Radio used to be a lot harder. On the communications side of things you could buy an expensive radio receiver and/or transmitter that required a skilled operator to use. At a lower level, you would be looking at choosing a specific band and dealing with things like modulator, mixer, and filter design, along with plenty of roadblocks to manufacturing which would also lock you into a specific application.

Software-Defined Radio solves some of these problems by allowing you to control how the radio hardware functions based on software. The advent of this has also been boosted by the availability of inexpensive hardware produced at scale. It is not the end-all of radio, but it makes the problem easier. That has led to wider adoption but we think what has been seen so far is only the tip of the iceberg.

Seen here is the USRP N210 which is a professional tool used by hardware developers that work with RF in their products. This tool proved to be so popular that National Instruments bought designer Ettus Research and now incorporate the USRP with their LabVIEW systems. The midrange USRP-210 model is a very capable SDR, operating DC to 6 GHz.

You Can Change It After It’s Built

Florian Fuchs/Wikipedia/CC-BY-SA 3.0 [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Florian Fuchs CC-BY-SA 3.0 via Wikimedia Commons
The whole point of SDR is less need for specialized hardware. One module can address a wide range of uses, even those that are currently unknown. Building and shipping hardware has high overhead, but formulating and distributing software (or firmware) updates may have much lower associated costs. Devices communicating using SDR don’t lock a platform into one specific set of communications. For instance, if you sell a base unit and multiple remote units, switching up the communications method in version 2 could render older hardware useless. You will have happy customers if they can can continue using their old accessories after a simple upgrade. It’s entirely conceivable that such upgrades would be pushed over the air (like from a base unit) as is seen with many smartphones.

The multiplier is, of course, crowd-sourcing development. One forecast of the future is a connected world. If device firmware has been released as Open Source, a motivated community will find a way to make that hardware even more useful.

In the next section I’m going to talk about the DVB-T dongle seen here. But one important thing to realize about it is that the chip inside this device is an SDR and is already in use commercially. The versatility of the chipset inside proves the point that SDR is a viable choice in consumer hardware. I’d love to see reliable numbers on how many of these have been sold to watch television, versus to tinker with SDR. Either way it’s great for the companies churning them out.

Start Learning for a Few Dollars

Don’t be ashamed if you know next-to-nothing about all of this. That’s where most people stand, and you don’t have to spend big or know much to dabble in SDR. Let’s face it, wireless communication is as close as a pragmatic mind will get to calling something “magic” and that makes SDR a delight.

Starting Simple

The thing that really turned my head was the advent of what is known as RTL-SDR. This is the practice of using television tuner USB dongles for Software-Defined Radio. That’s right, these “DVB Sticks” are made to watch broadcast television on a computer but inside is a Realtek 2832U.

SatNogs satellite receiver is based on a DVB-T Dongle and SDR

Connecting the dongle to your computer and launching some software allows you to listen in — both audible signals and transmitted data — on all kinds of things. We’re enjoyed reading [Dr. Droopy Nayhey’s] SDR guide on Hackaday.io because he’s taking this route. $12 in hardware (plus the computer and cables to be fair) and he’s tracking aircraft, listening to emergency band, FM radio, and “treasure hunting” for all the things in our world that are transmitting.

Don’t be afraid of this, these are receivers-only so you need no license or prior training. We’ve seen these morph into automated airplane filming rigs and you could end up adding to the flight tracking data network of FlightAware. The Grand Prize winners of the 2014 Hackaday Prize even built a satellite receiving station around a DVB dongle! See that little black stick centered vertically? Satellites do transmit information back to earth, you just need to listen for the data.

For getting started, and well-targetted applications, these dongles are a good option. But they are limited from around 22MhZ to 2200Mhz depending on which particular dongle you have. Going beyond those limits requires a jump to different hardware.

Getting More Serious

Parts that make up PortableSDR
Parts that make up PortableSDR

Earlier I said that SDR solves some problems but certainly not all. One device can’t rule all RF communications (yet). So those getting a bit more serious look to purpose-built SDR rather than piggy-backing on those TV receivers. This is still better in many ways than radio equipment of yore, as these boards boasts a highly versatile set of features.

Here we see an interesting take on SDR which placed 3rd in the 2014 Hackaday Prize. PortableSDR does away with the need for a computer to drive the software side of things and puts the circuitry in a durable case with a dedicated display as part of the user interface. It is aimed at people who are getting more serious about amateur radio, but as it stands is still a receive-only instrument.

On Monday we made an appeal for a Cinderella-story finish for the PortableSDR Kickstarter. I’m still hoping that this one makes it as I do believe it’s part of the modernization of the amateur radio movement.

Another example of that rebirth is SDR equipment specifically designed for amateur radio operators. We’ve been watching one such build as it progresses. This one centers around a Softrock SDR board which is controlled by a Teensy 3.1 and again, has a dedicated user interface that requires no computer. Notice the convergence here between traditional ham radio skills and the hacker movement?


Need I say more? There’s a growing movement of people who are playing with SDR. That will lead to interesting new applications and I believe it will eventually drive consumer electronic design. But if you need more inspiration, just look at the kinds of things people are building around SDR and make your own predictions.

[Featured image source: HDSDR.de]

113 thoughts on “Why You Should Care About Software Defined Radio

      1. He really did make this easy didn’t he! For instance, forgetting to end sentences with a period, or spell checking to see that breeze is spelt with only two e’s rather than 3. I’m not one to criticize other people … after all, I’m far from perfect, and if you read over my stuff you’d find lots of faults, but for Pete’s sake if you’re going to be the first one to throw a punch …

    1. HAD does a great job and I usually find at least one typo in every article. It is a good idea if you want to track your content across the internets and what not. In that way you know who is using your content without paraphrasing just plagiarizing. Just saying, its as good as a watermark… as long as the spider crawls it.

          1. Yep – I have a Toshiba remote which is ultrasonic. Similar sized transducers to what we find today in PING))) sensors.
            As far as early remotes go though, my father had system that predated the space command. He’d yell for me to get up and change the channel.

        1. We had the Zenith space command when I was a kid, and my dog’s collar with the license tags happened to rattle at the same tone as the “channel” fork. So, we had 2 remotes, one for us and one for the dog.

        2. ha- that reminds me of our first floor model TV that used that type of remote. It had 4 buttons- Ch and volume up/down.
          They were spring loaded and fired off a little tuning fork.
          The funny thing is we could flip the clasp on a ski boot to change the channel when we couldn’t find the remote.

        3. Yeah, I remember my teenage days coming home late in the AM and throwing my keys on the counter which in turn “tuned in” or turned on the TV full of static waking parents up so they knew what time I came home. Those were the days.

      1. I thought the same, but after consulting wikipedia now I think that OP is correct: “Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light.”

        So gamma-ray, x-ray, ultraviolet, visible and infrared are not radio waves.

    1. I have a Computer Science degree and I work in software. I love reading hackaday because it is a whole new world, a dazzling place I never knew, but when I’m way up here, it’s crystal clear, that now I’m in a whole new world with you. And little things like that difference help me get into and interested in this hardware stuff.
      With that said, there is obviously some boundary and some assumptions to be made about your audience, but I read that comment and took a moment to reflect on it and think through the difference. I am also the guy who cursed out loud when I realized I had to go out to the store and buy a new resistor I didn’t have instead of just finding one on the internet and copying it and tweaking to my needs.
      I may have been drinking while writing this.

  1. As long as the SDR uses a 100% open protocol and software so it CAN be modified… I’m all for it. The problem is commercial radio makers will try like hell to keep all of us from doing anything with it.

    1. SDR is a concept, not a protocol or software. You cannot patent a concept, nor copyright it.

      It’s like claiming carmakers will try to forbid others to make cars. The concept of a board with three or more wheels on it, a steering contraption and an engine providing propulsion is so generic you cannot patent it.

      All you need is a copy of GNUradio and a $10 USB DVB-T stick.

  2. Funny, I’m old enough to remember when superheterodyne receivers were still relatively new and flashy exciting design breakthroughs (well, plus or minus a decade or so)…now, in light of the SDRs, they look as old and tired as the TRF receivers they replaced.

      1. Superhet was towards the end of WWI, not before. And because of the cost of the extra stage, domestic superhet receivers didn’t really appear until the 1930s. I’ve got a 1935 Invicta radio that’s a superhet, and it works extremely well (I’ve replaced a few dried out capacitors, but all the 80 valves are fine).

        But yes, receiver technology does tend to move on with fairly bit jumps in design. SDR itself has been around for some time (I certainly attended lectures detailing it during my electronics degree in the early 1990s), but it’s the advent of cheap mass production devices such as these DVB tuners that have made it accessible to hobbyists.

  3. Those who’d like to see SDR in action might want to visit a Dutch website that hosts an online SDR HF receiver:


    There’s a lot of spectrum to tune in, but if you’ll like one that more interesting than most, try 14300 KHz USB in the ham 20-meter band. There are often nets meeting there.

    Articles like this one make me wish there’d be a joint website hosting hackaday with dollaraday—or whatever you want to call one where small businesses bring hackaday products to those of use who like to dabble but don’t have the time to hack. I’d love to have an inexpensive HF-VHF SDR that’d attach to my iPhone via an iOS app, and give me features like that website.

    That and a compact, single conversion PSK transceiver for 20 meters. It’d fit in a coat pocket, run off AA batteries and, mated to a smartphone, give worldwide communication with no intervening hardware required (unlike the Internet, which a repressive government can shut down).

    1. There are already ways to use a RTL dongle on an android tablet as I recall. Not sure if the same is true for ios though.

      Oh and that site you linked is from a technical university, in case you did not know, so it’s somewhat robust.

    2. Well there is the PortableSDR project. I’m glad to see it coming along. In the mean time, I’ve connected my RTL-SDR to my Android phone using a USB-OTG (USB-on the go) cable and run SDR Touch. It gets me my quick fix of SDR on the go. ;)

    3. 14300KHz in the ham bands is the Maritime Mobile Net that doubles up as an HF Hurricane Net during hurricane season. The net interacts with ship and land stations to relay messages to family, friends, and maritime rescue as needed, switching from “ragchew” and basic info to a more Emergency oriented mode on occasion.

    1. Although it’s indeed nice and nice how technology makes such things easy, with the RTL sticks it did require some lucky finds by hackers to discover it could be used like that. and it was really not designed with that in mind and they are rather protective with information on the chips.

    1. Chris has been rather busy, and hasn’t updated his site (or blog, for that matter) lately. From what I remember from a recent post on one of the mailing lists I’m on, they have reached the third iteration of the hardware (Whitebox Charley) which should be going for prototype production soon; Bruce Perens will have a working version of the Bravo board at the Orlando HamCation this weekend, for those that want to see it. I also think Bruce and Chris will have them at Dayton in May and the TAPR/DCC in September (location to be determined).

  4. As a hamradio guy who has always enjoyed building my own gear for the last 58+ years, SDR is the best thing since sliced bread. I started with Softrocks (kb9yig.com) and moved on up to WB6DHW’s UHFSDR –> HiQSDR transceiver (hiqsdr.com) –> Hermes transceiver (openhpsdr.org) –> Hermes-Lite transceiver built for ~$150 (hermes-lite@googlegroups.com and github for all the plans, software and hardware).
    Along the way I used RTL-SDR on the 2m and 70cms ham bands. I aso have an HF-upconverter I built for it but have yet to test it on HF – too many other interesting projects on the go.
    SDR is so much fun that I describe my commercial gear as being here just to keep the dust off the shelves.

    1. Er, no. It’s radio, right… only instead of the decoding done by hardware… it’s software!

      It doesn’t matter WHAT software. The point is, the signal that comes into the PC is not extracted audio, it’s pretty much a radio wave. Converting that into sound is done by software, because modern CPUs are ludicrously fast.

      It’s not a particular specific radio, it’s a concept.

    1. Well, sort of.
      FM “stereo” is interesting. It is actually two radio signals. Hold on though, it isn’t what you think.
      One channel is Left+Right audio mixed into a mono station. The next station is Left Only. Your stereo radio actually picks up both channels and then deletes the left audio from the mixed channel to give you the Right channel.

      So, an SDR can be configured to pick up the stereo signal. It might be a bit tricky to actually convert it to 2-channels.

        1. Actually SDRsharp has a separate stereo button when selecting the WFM setting.
          With weak signals sometimes it’s handy to be able to turn off stereo on FM.

          Now if other software has a stereo option is dependent on the developer, and I fear there are several packages without that option.

      1. One channel is Left+Right (the sum), the next channel is Left-Right (the difference), hence the multiplexing to derive both halves of stereo. Interestingly the Left-Right stereo subcarrier (centered at 38kHz) is amplitude modulated (AM), minus the carrier. Yes, that’s right kids, in FM broadcasting we use AM to give us FM stereo…

        *mind blown*

    2. I’ve used RTL-SDR’s to receive BBC Radio 4 on 92.7 MHz FM and stations within 88 – 108 MHz, VHF/UHF FM/SSB/CW on the 144 MHz and 440 MHz ham bands and higher with suitable antennas.

      The mode is determined by software only unlike an analog radio which would need separate tuners, filters and demodulators implemented in hardware for each mode.

      The software processes the mathematics equations that are required to produce whatever mode is needed.

      The GUI will have buttons for the modes – you want AM, click the AM button, FM, click the FM button, CW, SSB Upper or Lower Sideband likewise. A new mode being introduced only requires a software update.

      The panadapter display shows all the signals within the slice selected so you can click on any of the signals in view.

      Additional tools like a signal generator, Spectrum Analyser and Vector Network Analyser – a SDR transceiver is capable of performing those tasks also.

      1. AFAIK, the HD Radio codec isn’t even public. The lower layers are published, though, so you could probably decode the RF signal into a bit stream, and maybe de-multiplex content streams out of that, but you won’t be able to convert the content streams to audio without some significant reverse-engineering.

  5. Very good article, Mike. Very informative and well thought out.
    One other site that I visit every now and then is http://www.rtl-sdr.com/ , which is an ongoing blog with a wealth of information and cool things to try with your RTL-SDR dongle.

    I use mine as a bandscope for ham radio, as an FM radio receiver, and as test gear when needed (for relative, not precise measurements). I’m also still working on the Junk-Box SDR, which is an RTL-SDR and a Ham-It-Up stuck into an old steel case, along with a few other things from my junque pile to improve it (hopefully).

    Oh, one other site that’s worth a look, especially if you’re in the market for an RTL-SDR dongle: http://www.nooelec.com/store/ . They have a number of SDRs, upconverters, and accessories available at reasonable prices.

  6. I’ve been using a HackRF for a few months now and greatly appreciate how it has replaced the whole boxful of other radios and scanners I have – especially as most of them were only connectable to a computer via analog audio.
    It’s fun just to see what signals there are filling the airspace around you every minute of the day. And even more fun to home in on the odd ones and try to extract information from them.
    Naturally the PortableSDR appeals to me and I’ve been following its progress, but I’m afraid it is just too expensive and doesn’t offer me anything over the HackRF (I don’t need the portability). I wish I had the money to back it anyway, but I don’t.
    SDR has also led me into the world of DSP; another rewarding field of study.

    1. Search on google for RTL-SDR dongle, any that are defined as such will be OK, e.g http://www.amazon.com/RTL-SDR-DVB-T-Stick-RTL2832U-R820T/dp/B00C37AZXK 24MHz – 1700 MHz coverage.
      For short wave you would need a RTL-SDR upconverter as well. The dongle covers typically 50MHz – 1700MHz.
      Check http://rtlsdr.org/.
      Under Linux – http://napan.ca/ghpsdr3/index.php/Main_Page for ghpsdr3-alex for which you will build and use
      rtlsdr-server to talk to the dongle and dspserver to give it network and internet visibility, also QtRadio which is the GUI. Access via an Android phone or tablet is possible.
      Another application is Linrad.

    2. If the laptop has stereo audio in (it may not), a quadrature downconversion SDR that generates in phase and quadrature (90 degree phased) audio IF signals for processing in software may be less demanding on an older machine and it is fairly cheap and easy to build a receiver of this architecture:

      i.e. softrock inspired HF receivers




      USB TV tuner dongle RTL-SDR based software may work on an older machine anyway if the sample rate selected is sensible – an older machine may struggle with the maximum 2Mb sample rate.

      Check out ghpsdr3-alex as well, you may be able to use the laptop as a server and use remote clients to access the laptop radio server.


  7. Great summary Mike. The power of the dongle SDR is being able to see a spectral display of approximately 2 MHz in width, along with a waterfall. If you see a peak on the spectral display, a mouse click moves you to that spot and you can listen to what is there. I have been into radio comms for more than a half-century and the dongle-SDR at less than $20 is about the best value for the money a tinkerer can find. It puts wide band reception into the hands of the average, possibly financially challenged radio adventurer. The listening possibilities are endless. You can even use it to some extent when aligning receivers and transmitters. Kudos to all those gifted individuals who took a simple dongle used for TV reception and transformed it into wide band receiver. And thanks to all those who are constantly coming out with new innovative products in this field.

      1. Here’s a question for you- I would like to get into SDR, Could these things detect the signals from old 900MHz analog cell phones? How about the signals from those cheap 433MHz data transmitter boards from China? Lots of RF out there in the world and I would like to see and hear it.

        1. I can’t speak to the 900MHz cordless or cell phones, but 433MHz is no problem… google RTL_433 which comes with a few “decoders” to reveal what’s in many low cost transmitter/receiver pairs (weather displays, etc). Track planes’ ADS-B transmitters (at 1090MHz) with ADSBsharp and ADSBscope.

        2. An RTL-SDR can pick up any signal that would be receivable at your location anywhere between 24 MHz and 1700 MHz if you have the most common type of RTL-SDR (RTL2832 + R820T chipset) and a good antenna. With software such as SDR# (my personal favorite), you can take a good look any of those signals.

          Will it DECODE cell signals? Definitely not. Also, doing so in the US is technically illegal. Things may be different if you live in another country, so check your local laws.

          As for decoding the 433 MHz data? I’m not sure. You can certainly receive what they transmit, but you will need to know the encoding scheme and find/create a program to decode that (DSD maybe?) in order for that data to be useful.

          433 and 900 MHz are in significantly different parts of the spectrum, and have different antenna requirements. However, a discone antenna may be broad-banded enough to suit your needs, and they can be constructed at relatively little cost with materials that are readily available in most places.

          1. No, not cell signals, he’s talking about “cordless” land line phones – the kind where you could take the receiver around your house, and it would chat with the base station at 900mhz, which is the real interface to POTS.

          2. So is it continuous tuning from 24MHz to 1700MHz with no gaps? Yes, I was actually talking about analog cell signals. I have a pair of cell site transceivers for the old analog 900MHz cell band the preceded the 850MHz/900MHz GSM band They are before the time of most of the readers here. But 900MHz cordless phones are another interesting application.

  8. I’m a ham radio guy and my sdr dongle is a fun part of my shack. It gets stuff my other radios cant even think about touching. with some addon mod files it can do digital modes and airplane beacons. In fact im getting a new 25 foot mast tomorrow to hang up my st-2 scantenna love tax return time

  9. I haven’t seen anyone mention ATSC (as is used in North America), and the DVB-T sub $10 dongles don’t seem to support that. I know they should support both, since that should be a software thing. What am I missing? Is it that parts of ATSC are patented, and so, in order to add that to the software, they would have to pay royalty fees?

      1. There are situations where there is only a laptop or PC is available – no TV set. They work fine for the intended purpose of receiving TV broadcasts.

        DVB-T standard is widely adopted in very many countries across the globe and was in normal use long before digital TV was adopted in the States. ATSC is the main digital TV standard in the USA and I think is a follow on from the analog NTSC. .
        They are different standards and not compatible.

        I suppose someone could develop SDR to work using ATSC dongles but no one has done so to date.

        RTL-SDR using the DVB-T standard was developed by a European working with current European tech.

    1. The big difference is not a software thing, but hardware. The ATSC signals have a bigger bandwidth (6Mhz) than the dongles support (2Mhz). The dongle can receive the signal, but only in black and white with no sound.

        1. Check out NutsAboutNets’ dongle spectrum analyzer, allows you to see up to 300 MHz of spectrum at a time. I bought the package on Ebay last year and am having a great time with it. Very inexpensive and has enough options to satisfy basic spectral scanning for the hobbyist. You can easily see the spectral display of OTA digital TV channels.


      1. And there ARE some $50 ATSC dongles that work with linux. These would have a larger bandwidth, which could potentially make them more useful (particularly if utilizing some type of spread-spectrum system).

        1. The trouble is that most dongles don’t have any way of getting the raw samples out, so they can only be used as TV receivers. The RTL-SDR dongles actually support even wider bandwidths of up to 8MHz in their intended use as DVB-T receivers – more than ATSC – but you’re restricted to 2-3MHz of bandwidth when using them as SDR receivers because it’s kind of bolted on as an afterthought.

  10. I bought the dongle, i installed gnu radio on my laptop and i stuck then because i thought i would have a ready to go program, and what i get is a work sheet to design something like a circuit with block of data.

    Well i’m an informatic, but this sdr topic is new for me, i have a ham radio, but only a few knowns in eletronic, and i would like to get it (the program) working. Any help? tuturials or anything that teach me how to create the gnu radio circuit to be able to listen some frequencyes?

    Or any other ready to go sdr program?
    I must say i’m a little busy and i did not search on google yet more than 2min.


    1. For gnuradio there is gr-osmosdr.
      If your platform is Linux there is gqrx, gr-osmosdr, ghpsdr3-alex and Linrad.
      Ubuntu supplies gr-osmosdr and gqrx-sdr, the others you would have to build from source.
      openSUSE doesn’t supply them and I haven’t checked Fedora or any of the other distros I run.

  11. As far as digital over the air TV signals in the USA, you cannot use the dongle to decode them and watch OTA digital TV but you will be able to see the prominent ATSC “spike” and the wave form of a digital TV channel which is about 6 MHz wide. The spike appears at the beginning of the wave form for each channel.

      1. Well since these dongle SDRs were originally designed for DVB-T (terrestrial television reception) in certain countries I would think that the answer is yes. Am not aware of any software for digital OTA (Over The Air) television reception here in the USA. The dongle certainly has the frequency range for OTA signals.

        Would not mind being able to receive TV on my computer, with an old-fashioned TV antenna on the roof.

  12. Ok, my limited understanding of radio – based on a crystal radio kit I build as a kid – is that antennas are tuned to a specific frequency.

    So how does one antenna sample an entire 20mhz wide band (or whatever), like that WebSDR site? The rest of the DSP stuff makes sense to me except for that bit : )

  13. In case it hasn’t been noticed, but the PortableSDR kickstarter was successful… 66K (goal of 60K). I’m hopeful for many good things to come of that project. Congrats to all involved for the kickstarter success!!!

  14. Some Questions?
    What are the possibilities for SDR at 5-55KHz (metal detector frequencies)
    I realise that the PC sound card is a possibility but that is not very portable
    perhaps just a coil antenna – amplifier then AD on arduino or similar.
    How does one know before hand / guard against a overload & destruction of the A/D converter? if feeding the signal out of an amplifier.? what sort of amp (audio?) would be suitable
    anuy other suggestions for a low cost hack to look at signals in this range.

  15. speaking of IR remotes and radios. . If you put an IR remote near an AM radio and push buttons, you can pick up the pulse that it is sending in IR. I’m bnot sure every single IR remote does that, but every single one I ever tried did. If you ever see those $1 universal remotes at dollar tree, it is guaranteed to do that. sounds pretty cool too, depending on which universal remote code you have set.

  16. I like the trend in technology for us to be able to “see” more and more of the universe. In this case, a much wider range of the electromagnetic spectrum rather then just little slices for specific uses.

  17. Personally I still prefer I.F. coils, potentiometers and tuning capacitors in a hand built unit that one can build, modify and repair one’s self, but then I was born in 1946 so am probably viewed as a technological dinosaur by the general readership :-) VA7DB Vancouver Canada.

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