Controller for Analogue Sampler
Ok This is really here as a favour to someone who wanted to know how to use an LM567 tone decoder. Since I had just such a circuit I've gone to the trouble of converting it up to a GIF and posting it here for all to see. The explanation of how it works may be a little "lite-on" but it can't be helped. I just don't have the time for a better explanation.

So first you say. "Analogue Sampler?... Hu???" Ok let me explain. The term was derived from a joke David Vorhaus pulled on a reporter once. For those who don't know, David Vorhaus is an electronics technician by trade but is one of the few true synthesists. Inventor of such things as the Kaleidophon. A four ribbon synth controller that looks something like a chapman stick. He'd just released another album heralding the invention of sampling as common place. He said he only releases an album whenever there's been a significant change in synthesis technology. Anyway he said to the reporter something like. "Of course we had samplers back in the early 60s you know." The reporter sort of went "oh yea?" in disbelief. Vorhaus went on something like. "Oh yea sure. You could sample for anything up to 90 minutes at a stretch with no quantize noise and very reasonable quality in general. Some even had valves in them. " As you can imagine the reporter's jaw hit the floor. Imagining this huge machine that filled a building with wall to wall vacuum tubes and the power consumption of a small city. The reporter composed himself and said "So what did you call this thing?" Vorhaus replied "it was called A tape recorder."

So based on this we called the device an Analogue Sampler. It was based round an old 4 track cassette deck. The idea was that it would use the 4th track as a control track. We'd record our backing tracks on the first two tracks in stereo and a click track for the drummer on track 3. The tape would first be initialized with a tone. When the tone was lost the tape machine would wait a short period and stop. At least I think that's how it worked. It was a long time ago. It could have run till it picked up the start of the next tone but I'm not sure.

In any case the problem was that it needed to be absolutely sure to pick up the right tone and not be fooled by any noise or spikes etc. The kind of things that happen in a live environment with lighting rigs and such. It had to be absolutely fail safe. Well as fail safe as I could make it. The first attempt used a filter and it wasn't too successful over all. The circuit here is the second attempt. It used a 567 tone decoder. It produced the tone as well as detected it. A series of flip-flops and other logic was used to control the motor of the tape machine and eventually the whole unit was mounted inside the cassette deck itself. Simple RC networks provided the delays and it could even cope with short but complete tone drop-outs caused by damaged tape. It could pick up the tone even when attenuated by 30dB. And of course it would refuse to be confused by levels in excess of it's designed range. It was almost impervious to noise. Even when the tone was recorded in the noise floor of the tape. It was pretty remarkable really. I don't think I've ever built anything more bullet proof.

The circuit says it all. Well almost all. The rest can be gathered from the data sheet on the LM567 tone decoder. Of which can be obtained by going to National Semiconductor's site on the web and downloading it in Adobe Acrobat (PDF) format. The neat thing about this circuit is that it not only decodes the tone, it also produces it in the first place with it's built in oscillator. The tone is gated on every time the start switch is pressed. That way the same circuitry is used in play or record. Pressing stop gates off the tone in record mode and then stops the tape. That way the time delay is roughly the same in both modes. The op-amps are not critical. I used what I had at the time. Except for the 3140. That is critical for squaring up the data into the tone decoder. Unfortunately 3140s tend to be somewhat on the expensive side. If anyone knows where I can get a cheap tube??? The only other thing possibly worth mentioning about this circus is that you need to run it on 10 volts. I believe that's the maximum safe operating voltage for the 567.

There was also a MK III Analogue sampler. This time I was getting a little too ambitious. It was based round a Z80 computer. Complete with colour graphics display. It had two tone decoders and also a 600BPS modem. The basic system worked in the same way as the MK II version however it also recorded a data track along with it at a different frequency. The data would have been used for event control. There was provision for converting it to MIDI events which could have been used for control of lights or anything we needed. Of course there would have been a limited number of events with only a 600 baud modem but useful none the less. It was also thought that even lyrics could have been encoded in such a way as to provide a kind of auto-cue system.

Of course it never happens that way. We bought a DAT machine and the AS got scrapped. Hell I can't write software for peanuts and besides, the DAT machine did most of this stuff anyway. With digital quality no less. I've not included the circuits for the latter generation of AS because I can't quite work out how it was all spoze to work after all this time. It's really complex by the looks. Has a couple of audio transformers to isolate the control from the audio and a UART and stuff. But I'll post the stuff if anyone's really, really interested.

analogue Sampler control circuit View or Download
Schematic in
GIF file format