If you are fortunate enough to have had the opportunity to play with an analogue-reel-to-reel tape recorder in a well-equipped studio, you probably looped the tape around to create an echo, or a delay in the audio. It was a desirable effect to have, but not a practical one for a guitar pedal or similar portable accessory. Silicon alternatives for creating delays have been in production since the 1960s, first the so-called bucket brigade delay lines that used a switched chain of on-chip capacitors, and more recently all-digital chips that process the delay by storing samples in RAM. One of the more popular of those is the Princeton Technology PT2399, but it comes with something of a snag for the experimenter in the form of a sparse data sheet. Thankfully the folks at [Electrosmash] have come to the rescue on that front with a thorough technical examination of the chip that should fill in any gaps in the official documentation.
After a brief examination of the range of chips of which the 2399 is a part, they dive right into the chip’s internals by rearranging the internal circuit diagram from the data sheet to the point at which it makes more sense. At which point the difference between the chip’s delay and echo functions becomes obvious, through the inclusion of a feedback path.
We then are taken through the pins, examining what lies behind the power supply and analog inputs and outputs. We are somewhere between a data sheet and an app note here, as some of this is information rarely present even in really good data sheets. Finally, we are taken through the chip’s performance, with real-world distortion and noise measurements. Armed with this page, the would-be PT2399 designer really can say they know what they are working with.
Surprisingly few PT2399s have appeared on these pages, however one did pop up in the Synthbike.
The Panasonic MN300X series was the last BBD bucket brigade devices produced. Then restarted by cool audio in limited production, then lost to history again. Would be nice to hear a comparison between this chip and the MN300X series for comparison.
You should have mentioned the reverb units that came before the bucket brigade. They used long springs to delay the sound. When you drove over a bump you’d get a bong sound.
Spring reverb is alive and well – still used in some amps – although digital is now accurate and cheap enough to replace it. The Belton reverbs use several PT2399’s and there are a number of reverbs based around using many PT2399’s with different delays and feedback. I haven’t heard one I particularly liked but they aren’t bad!
I got several of these in a pack a couple of years ago and they are cheap and cheerful – surprisingly good for a faux tape echo (the darker fizzier kind of analog echo). And this kind of detailed information will help make more interesting effects (think modulated delays) more accessible to DIYers. good job!
“I got several of these in a pack a couple of years ago…”
Just
google‘DuckDuckGo’ (makes a much nicer verb, doesn’t it?) the string “pt2399” and you’ll find enough info to solve all your insomnia problems for weeks– including the fact that these are STILL available from Amazon in packs of two, ten, and twenty…if you still feel the need to buy packs of them.“Surprisingly few PT2399s have appeared on these pages, however one did pop up in the Synthbike.”
I think Intel has demonstrated how important documentation is.
I saw a studio reverb unit that had a record head and the maybe 10 playback head. The playback heads could be moved closer or farther to the record head to “dial in” the exact desired reverb delays.
There’s an excellent PT2399 kit available on AliExpress for <$5, it’s sold as a karaoke echo kit, red PCB labelled orchid audio. Also has a ne5523 op amp. I’m slowly drawing up a schematic for it as none is available. It uses a 5.1v zenier to supply voltage for the PT2399 so a well thought out design. Easy to replace the resistor on pin 6 with a 30k or 50k pot and you can control echo length.
Measure the phase and apply it to the delay so it’s always canceling itself out. Put several in a room microphone to loud speaker and you’ve got a mute button.
That’s very cool. An audio delay line plays very heavily in my HaD .IO project of a Single Frequency duplex Repeater. The basic idea is a separate radio transmitter and receiver on the very same radio frequency can be toggled on an off at a rapid speed rate. But to repeat the receiver’s audio to the out of phase transmitter an audio delay line like the PT2399 is needed to store the audio for a millisecond before dumping it into the out of phase transmitter audio.The rate is so fast that there is no noticeable chop in the audio stream.
So a mobile simplex unit can be transmitting on channel 1 while everything he is saying is being re-transmitted like a double keying from the stronger transmitter on channel 1. No hetrodyning or feedback is present in the resulting audio as technically the receiver is phase blinded to the out of phase transmitter. This weird technique was invented by Motorola in Vietnam War. Cellphone technology exploits it today. Works great for radio obstacles like mountains and buildings.
https://hackaday.io/project/12348/gallery#286629f9cca4e6bbcc985d8ec03bcebb
Isn’t that a chip that is inside Monotron Delay?
Yes the monotron has the smd version of the 2399
Could a linear CCD (as used in scanners and spectrometers) be used for this? Each line of pixels on a CCD works in a bucket-brigade fashion, kinda. You’d need to find one with an electrical input, or somehow apply light to just one pixel as the input. Then you’d just run it at a much lower readout frequency than normal, and not connect its output to an ADC.
Yes you can do that. I’ve done it with an audio modulated laser.