Re: Crystal's new chip

At 04:34 PM 7/7/97 -0500, you wrote:
>
>> Easily done with pitch bend...
>
> Ahhhhh, I'm obviously a bit out of my league here. I've never touched
anything MIDI at all. I'm not even sure what pitch bend is....

It a control that's sent along with the note value to tell the synthesizer
how much to flatten / sharpen the note. The increment is quite small
allowing one to slide from one note to the next, there's no perception of
"steps" during the slide, sounding much like a slide guitar.
>
>> No need, you're missing the beauty of the synthesizer chip. It has pitch
>> bend. I can't remember what the midi specs is, but the resolutions is real
>> high. Something link 4096 or 2048 steps between half notes! Whether it's
>> done with a PLL (most likely) or whether they just tweak an analog clock
>> inside the IC, I don't care, the point is you have control of the frequency
>> of the played waveform with very high resolution.
>>
>> If you find someone with a midi synthesizer, hooked up to a midi keyboard,
>> and press a note and play with the pitch bend control you'll see what I
>> mean. You can't here any stepping of frequency between one note and the
>> next, even though it's being fully digitally controlled.
>>
>> As far as stepping through the wavetable ourself, I don't think that would
>> be possible. The table needs to be accessed by the synth chip in a truely
>> random way, in order to play back the 32 simultaneous sounds. And it's kept
>> in a "proprietary compressed" format. You might have access to that info, I
>> obviously don't, and don't need it. I would use the synth chip it's self to
>> save the samples. I'm not concerned with how it saves them. I'll just take
>> what ever data it saves in RAM, and ROM it.
>
> .....not stepping through it, just controlling how fast the chip steps
through it (i.e. the clock frequency.) My idea of a wavetable was that it
was just that -- a lookup table for values of the sound/wave vs time. You
can represent a sine wave of one frequency with a certain wave table. You
can represent a sine wave of twice the frequency with the SAME wave table,
but stepping through it twice as fast....This sort of implies that the
signal be periodic, because if you want the signal to last any significant
length, you just keep repeatedly looping through the table.

I'm sure that's what goes on inside the synthesizer, the cool thing is I can
just tell it what frequency I want a sample played at, it calculates all the
step frequencies needed to play the sample at what I've asked for -- all the
way down to what ever micro steps are needed to play in between samples (it
probably even does a nice interpolation between the samples when needed).
>
>> All that *and* wavetable synthesis. Looks like a very nice chip, they told
>> my friend that they would be available near the end of the month (or maybe
>> they already are -- I'm having dinner with him tonight, I'll find out), but
>> we're definitely going to grab some.
>
> Hmmmmm, maybe I'm misunderstanding what wavetable synthesis is (i.e. it's
not just a lookup table, which could be implemented on a DSP with RAM/ROM)
looks like I need to do a bit of "homework."

No, I'm sure they've implemented it just as you say. It just that the DSP
can access the table in such a way as to allow 32 different sounds to be
simultaneously played back (32 separate mux'ed pointers!). It also allows
micro stepping of the samples (most likely with interpolation) to allow for
very slight changes in pitch.

I'm sure it has the ability to play a single "once through of a sample" or
loop, since this is what the currently available synth chip, with the
builtin ROM, allows. (Ex: Drum samples are played once, while violins are
played as long as a key is pressed.)

After which the DSP does all the mixing, with the availability of reverb and
chorusing, and then does all enveloping! Makes everything almost too easy!

>> The external ROM looked very standard in it's interface. Just a basic
>> parallel data and address busses.
>
> It looks like this is a new chip. The one that I'm looking at has a
built-in ROM. Ignore the part number I gave you, then....

The one with the built in ROM is the one my friend has already laid out a PC
board for, it was while asking questions on that one that he found out about
the release of the new one.

-Zonn
Received on Mon Jul 7 15:02:41 1997