[sdiy] Frequency shifted from BBD?

Sun Oct 27 10:13:09 CET 2024

Isn't this just like applying Shepard / Barberpole to some BBDs, or am I
missing something?

>  I'm not aware of any multi-tap BBD

There was a multitap BBD (MN3011?) but with uneven taps for reverb =
unsuitable.  I vaguely remember some discussion about using multitap CCDs
way back.....

Steve

On Sun, 27 Oct 2024 at 04:49, Kylee Kennedy via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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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