[sdiy] Frequency shifted from BBD?

Sun Oct 27 05:42:23 CET 2024

Thanks for the insights Brian.
Always great to brainstorm these weird circuit ideas here in this group.
I also can't read the article you cited but it sounds similar to the idea I
had where the BBDs are in parallel to the audio source and a scanner +
VCAs circuit would go through the BBDs in round robin fashion. I was
thinking four BBDs would be sufficient and it's interesting they mention
just two and bouncing back and forth at the end of the 512 sample limit.

I was thinking you'd have the next BBD in the scanner loop start recording
halfway at 256 samples through the playback from the first BBD and
the clock would always be the same rate during recording and then when
the scanner gets to that BBD for playback it would shift to a second clock
at the new pitch shift speed. Starting from halfway through the samples
would allow more range to pitch up. Audio fade in may need a few
samples delay before starting to clean up any pitch shifting glitches.

I might try to mock up a breadboard version to see how it sounds and or if
it works at all as a pitch shifter. What could be an interesting mod is
each scanner BBD step could have a different pitch which is an effect I
like that's found on the H910/H949 Eventide effects.

Kylee

On Sat, Oct 26, 2024 at 2:07 AM Adam (synthDIY) <synthdiy at adambaby.com>
wrote:

> Wow, great find Brian!
> I can't access the paper, but the abstract says it was presented at the
> 42nd convention, May 1972, and that
> "A demonstration will be given to illustrate the state of art in speech
> time compression-expansion.
>
> I wonder if the demonstration was tape or BBD based? Maybe both?
> From your description it certainly seems to bring the BBD based scenario
> closer to the multiple tape-read-head systems capability
>
>
> A
>
>
> On 26 Oct 2024, at 2:56 PM, brianw <brianw at audiobanshee.com> wrote:
>
> On Oct 7, 2024, at 6:45 AM, Todd Sines <sines_list at scale.la> wrote:
>
> [...] Varispeech II (cassette version of the Varispeech 27Y) which is more
> like a time stretcher, in that you can alter the speech but keep the pitch
> the same (in order to decipher fast-speaking conversations)
> https://obsoletetechnology.wordpress.com/repairs/lexicon-varispeech-27y/
>
>
> Very interesting. This link lead to an AES paper from 1972 where the
> author described time compression and time expansion via BBD.
>
> http://www.aes.org/e-lib/browse.cfm?elib=1793
>
> I've never seen this technique used for BBD anywhere else, but the circuit
> actually counts the clocks and abruptly changes the rate when all the
> buckets are used. The BBD devices are used in parallel, where one is
> "recording" while the other is "playing" and then vice-versa. Furthermore,
> when the BBD is in "record" mode, it operates at a different clock rate
> than when that same BBD is in "playback" mode, but each mode has a steady
> clock for the counted number of cycles.
>
> Thus, for a 512-stage BBD, the control circuits count exactly 512 clocks.
>
> For "time compression" (pitching down), every 512 clocks, the output swaps
> between the two BBD chips. Meanwhile, the BBD that's not connected to the
> output is being clocked at a higher rate, so more than 512 clocks are
> issued, and some of the audio samples are lost because the output is not
> connected when those samples make it to the last bucket.
>
> For "time expansion" (pitching up), every 512 clocks, the input (record
> mode) swaps between the two BBD chips. The BBD that's in output mode is
> running at a higher clock rate, so it would empty all the buckets. The
> clever trick in this mode is to connect the input to the output to make a
> little tape loop. That way, when the buckets are emptied of new material,
> the output continues playing old material. The clever part is that the
> input is only connected to the output when the BBD is in "play" mode, and
> is switched over to the live input when that BBD goes into "record" mode.
>
> Seems like the BBD circuit described actually affects pitch in real time,
> while time compression or expansion would have to be altered by playing
> recorded material at a different speed using some other device, such that
> the BBD circuit can then restore the original pitch. I'm not sure why the
> author referred to this as time compression and time expansion, unless
> you're supposed to understand that it's only one part of the solution.
>
> All of the BBD circuits that I'm familiar with provide a continuous clock,
> with input always connected and output always connected. Pitch shifting in
> this configuration is only seen by varying the clock over time, and that's
> usually small, contiguous changes in clock rate rather than an abrupt jump
> (well, except for the one abrupt jump in the ramp wave).
>
>
> In a way, I suppose it's like comparing a BBD pitch shifter that operates
> via a VCO clock that's controlled by a ramp wave with and digital clock
> that's controlled by a square wave. The caveat is that the square wave
> method requires precise counting of clocks and abrupt synchronous switching
> of input and output connections, although that would be cheaper since no
> VCA are needed. The ramp wave method can completely ignore the precise
> number of clocks that happen to occur, and the transition between one BBD
> and the other can be handled by a smooth crossfade rather than an
> instantaneous switch.
>
> The AES paper makes me think of a Chamberlin keyboard "sampler" that uses
> BBD chips instead of pieces of tape, and one that's constantly recording
> new samples onto the tapes in a way that playback occurs at different
> speeds. If there were a BBD large enough to hold 8 seconds of audio, you
> could make a very unique Chamberlin this way (even without the pitch
> change).
>
> Brian Willoughby
>
>
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