Comments in line.
There's this preoccupation I have since the advent of going "digital",
let's say since I heard music being played on CD in the early 80s. I
grew up with access to electronics equipment that would generate
"square waves" in some sorts of analogue fashion, including originally
"digital" chips, even driven from frequency stable crystals and so on.
Correct, though most analogue synthesisers generate a sawtooth or a
triangle and then use a comparator to turn that into a square wave.
So, square isn't the only waveform.
In fact I built my own organ/synthesizer based on a top octave
synthesizer chip around 1980 which I gave CMOS divider chips to get
well symmetrical, pure and pretty undistorted square waves to a analog
mixing rail construction, and I must say (I was a teenager) I recall
the different sounds. the feel if you like, of all those different
square waves by themselves and some the filter and modulation
constructs I made quite well.
The other REALLY important thing to mention is that you can pretty much
guarantee that not all 12 waves were perfectly in tune relative to each
other.
This subtle detuning, or beating, makes a huge difference to the feel of
"analogue" Vs Digital, but it can be replicated
Now, like everybody else, I'm used to listening to a lot of audio in
some form of digital source format, ending up at one of the varying
types of Digital to Analog Converters, to enjoy digital music on for
instance a smart phone, a HDMI based digital stream converted by a
TV/Monitor, a very high quality DIY kit based converter setup,
standard computer and bluray player outputs (both not bad) and known
brand studio quality USB ADC/DAC units (Lexicon, Yamaha, and a Burr
Brown/TI chip based DIY kit) and finally from some variety of digital
music synthesizers (a.o. a Kurzweil and a Yamaha).
Though, remember these are mass market products, they will use the
appropriate part for a given price point.
Now, if you want a GORGEOUS sounding DAC, go play with a synclavier.
These are discrete DACs and sound like NOTHING I've ever heard before..
just utterly amazing.
The simple question that forced itself on me often, as I"m sure some
can relate, after having been used to all those early signal sources
including a host of analog synthesizers I had in the past, and a lot
of music in various analog forms from standard pop to G. Duke and Rose
Royce to mention a few of my favorites from an earlier era, is how can
it be that such a simple wave like the square wave, just two signal
levels with a near instantaneous jump between them, can be so hard to
make digital, if you listen with a HiFi system and some normal musical
signal discernment ?
because, what digital gives you is a "perfect" result, Analogue can
never do that, even the slopes will be slightly shaped and there will be
frequency drift and RARELY would the squarewave be precisely 50/50 duty
cycle.
The answer is relatively simple: a digital square wave for musical
application comes out of all current standard DACs with imperfections
that I recognize and have an immediate form of musical dislike about.
I think it's the wrong way round, they have the perfections you don't
like :)
Not that a software synth can't be put on, played and create some fun
with square waves, I'm sure it can to some degree be fun and played
with in some music, but for sound enthusiasts, all that digital signal
processing does come across as often the same sounding and not as
musical as I remember it can be by far.
Again, remember there is more than just a square wave to play with.
Is it possible to do something about that? I'm an univ. EE so im y
official background knowledge, there's enough to understand some of
the reasons for these sound limitations easily. Solving all of them
will prove to be very hard, given standard DSP and normal current
DACs, so there is that. To begin with the understanding *why* such a
simple "digital" square wave doesn't sound warm and nicely flutey from
a digital system in many cases: the wave as to be "rounded" to fit in
the sample timing, and the DAC essentially doesn't necessarily "know"
how to create those up and down signal edges with accurate timing.
actually, a lot of good "analogue synth" flute sounds were based on a
tirangle wave, rather than a square.
Can a DAC do a better job ? Yes, but not by just feeding it a pure
square wave, rounded to the samples. One could make use of serious
oversampling, and a much higher rate DAC, for instance I've tested a
very high quality DAC with adjustable type of built in "oversampling"
filter (low pass or short, hard window reconstruction) at almost 10
times CD rate (384k s/s),and surely this makes the sound more
acceptable. The monitoring and pre-amplification as well as the
analogue (electronics based) DAC filtering will matter for the sound,
too.
This is what I mean, you're creating imperfections from a perfect
source.
Now recently I've worked on quite a different type of problem, not
important for this sharing at the moment, which as one of it's
(complicated) side effects can produce components to a digital signal
that try to use the (limited) DAC filtering, usually some internal
up-sampling ("oversampling") with either a built in DSP FIR (some
short impulse with at least some low-pass qualities) or IIR (some
standard low pass response) to create a purer sounding square wave
approximation from a frequency limited digital wave source.
Anyone else worked on this to some extend ?
I would suggest considering variable sample rate playback systems rather
than fixed rate playback.
I also think there's a lot more looking at examples of the signals you
like that needs to be done, i.e. measuring drift, duty cycle, etc.
Paula
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