[sdiy] uLaw DACs in the modern era?

cheater00 cheater00 cheater00 at gmail.com
Wed May 23 12:13:32 CEST 2018


I think the glitches might contribute to the sound - but I won't be able to
say for sure without measuring.

On Wed, 23 May 2018 11:54 Tom Wiltshire, <tom at electricdruid.net> wrote:

> I don’t see the point of such exhaustive testing. We know well enough what
> the DAC does, since the datasheet tells us the exact levels in detail. As
> others have said, you can use a look-up table to get the same outputs from
> a modern higher-resolution DAC (14 bit would do, 16 is ample). As far as
> the u-Law encoding goes, the output from the AM6070 and the modern DAC will
> be identical. While there might be small differences due to different DAC
> glitches, I don’t think anyone has ever claimed that the *DAC glitches* are
> an important part of the sound, and such an effect would be utterly swamped
> by effect of the 8-bit encoding anyway.
> There may be other stuff that’s important after that (distortion in
> following stages, CEM filters on some sounds not others, etc etc) but
> again, these are not specifically anything to do with the DAC or the
> encoding.
>
> I find the encoding the most interesting part because it allows a much
> wider dynamic range from only 8-bit sample data, which gives the units more
> “punch”, while the low bit depth and extremely sketchy reproduction of high
> level signals gives a measurable (although not offensively perceptible)
> “crunch”. This is a good match for drums.
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> > On 23 May 2018, at 05:28, cheater00 cheater00 <cheater00 at gmail.com>
> wrote:
> >
> > That's what people did - anyone using those as their main drum machine
> would have them modified to take external eeproms, with a zif socket,
> usually in a side car connected by flat tape. People did that and samples
> played back through those still had that extra quality, but obviously that
> is subjective. The best way to figure this out would be to get some high
> resolution samples of common waveforms coming out of those - bass drum,
> pink noise, white noise, sine waves, triangle waves, square waves - at a
> few different frequencies (35 Hz, 65 Hz, 255 Hz, 1005 Hz (all frequencies
> offset by 5 Hz to prevent capturing mains)), and amplitudes (0 dBFS, -0.5
> dBFS, -3 dBFS, -6 dBFS, -24 dBFS, +0.5dBFS (or whatever reconstruction
> overshoot would allow). Record with at least 20 bit headroom (ie bit rate
> minus noise floor minus recording level below 0 dBFS). Then, look at it in
> a wave editor and sonograph, searching for glitches.
> >
> > What do you mean by layouts, Paula? I guess you don't mean top posting
> since you top post too.
> >
> >
> > On Tue, 22 May 2018 14:08 Tom Wiltshire, <tom at electricdruid.net> wrote:
> > Yeah, I’m not sure swapping the EEPROMs counts as “able to take external
> samples”, although it’s literally true - you *could* put whatever you
> wanted in there if you’re willing to do the work of preparing the samples,
> converting to 8-bit u-Law and burning the EEPROMs.
> >
> >
> > > On 22 May 2018, at 11:46, paula at synth.net wrote:
> > >
> > > Not sure what you mean by "take external samples", the LInndrum,
> Drumulator, and many others are fixed (unless you change the EPROMs).
> > >
> > > Paula
> > > p.s. I'm struggling to follow emails with layouts like this.
> > >
> > >
> > >
> > > On 2018-05-22 11:02, cheater00 cheater00 wrote:
> > >> Those drum machines take external samples, too.
> > >>   On Tue, 22 May 2018 11:00 , <[1]rburnett at richieburnett.co.uk>
> wrote:
> > >>     I suspect that a lot of the perceived "punch" in the sounds of
> early
> > >>     digital drum machines is down to the same sorts of processing
> > >>     applied to
> > >>     the acoustic drum recordings before sampling that we would apply
> to
> > >>     drums these days to obtain a more punchy and "produced" sound.�
> > >>     Namely,
> > >>     things like dynamic range compression with artistically chosen
> > >>     attack/release times, gating, dynamic EQ, etc.�  Oversights in the
> > >>     recording process like allowing a fair degree of clipping also
> make
> > >>     things sound a lot harder too.�  Heavy clipping is not uncommon in
> > >>     the
> > >>     samples of these early drum machines!
> > >>     -Richie,
> > >>     _______________________________________________
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> > >> References
> > >>   1. mailto:rburnett at richieburnett.co.uk
> > >>   2. mailto:Synth-diy at synth-diy.org
> > >>   3. http://synth-diy.org/mailman/listinfo/synth-diy
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