pt2399... any better info than the data sheet

[Brian Marshall]

the pt2399 data sheet seems to kind of be a little light on the info side.

i really dont have any understanding of what pins 7 8 9 10 11 and 12 do... I sort of understand the rest of it.

anyone have any info that is more comprehensive?

[Rob Strand]

I've been told by the manufacturers (and there's no other way to find this out) that pins 7 and 8 are used to smooth the a biasing supply inside the modulator (MOD) and demodulator (DEM) blocks.  There's no reason to mess with these pins, or the caps on it.  (Initially I thought this was part of the ADM slope control but it's not).

Pins 9, 10, 11 and 12 simply access opamp pins as can be seen from the datasheet.  What those opamps do is another matter.  You actually need to to have an idea how an ADM codec system is constructed to undestand these in detail.   In simple terms the opamps are wired as integrators, you add the external caps for form the integrators - the internal 4.7k resistors make these leaky integrators to prevent the signal levels getting too large.  OP1 is part of the ADM modulator, and OP2 is part of the ADM demodulator.  OP1 goes into the feedback loop of the encoder and OP2 sort of undoes what OP1 does in the feedback loop that's why you need the same R and C values for both OP1 and OP2 - that's probably the most important thing to understand.  At the end of the day, unless you know better, you probably shouldn't mess with these either.

The modulator/encoder part is a 1-bit A to D which pumps a stream of bits, representing the audio input,  into the RAM bank that is used for the delay.   The delayed digital stream then pops out the other end one bit at a time and the demodulator/decoder reconstructs the 1-bit digital stream back into analog (ie. it's a 1-bit D to A).  The delay is done digitally.  You really need to study about the subject to undertand this more.  That's how the chip works and you can't change that - it's debateable whether you really need to know how it works at this level.

[Brian Marshall]

i guess what i really wanted to know was if anything creative could be done with op1 and op2.  I didnt realized it stood for opamp...  Im not sure i understand what your saying 100% but i think what your telling me is that op1 and op2 cary the digital signal, and are integrated for some reason (perhaps to keep the ons and offs from interferring with the clock frequency??? i dunno)  I'm guessing that they couldnt put the caps in the chip because they are too big, or would interfere with the circuit somehow.

i actually found this data sheet while i was typing this... this seems much more comprehensive... not really complete, but better.

http://noise-room.com/sitebuildercontent/sitebuilderfiles/2399datasheet.pdf

[Rob Strand]

Im not sure i understand what your saying 100% but i think what your telling me is that op1 and op2 cary the digital signal, and are integrated for some reason (perhaps to keep the ons and offs from interferring with the clock frequency??? i dunno)

A bit more accurate would be the opamps (or more precisely the integrators) are actually used to rebuild the analogue waveform from the digital one.  

This is a gross simplification:  A "normal" D to A would take a number x and convert that to a wave height in analog form - the valve x requires a number of bits, say 16 bits.  A single bit D to A can't represent the wave height as a number, since it only has  0 or 1 to represent the height.  What happens instead is a 1 means go a bit higher than *last time*, and a 0 means go a bit lower than *last time*. ie it pumps the analogue output up and down from last time.  In order to know where you were last time you need an integrator - the integrator is used an analogue memory.   There's actually a bit more to it than that but it should at least give you an idea how 1 bit D to A's work and the fact you need an integrator. The A to D ismore complicated to understand but is based on a similar idea.

I'm guessing that they couldnt put the caps in the chip because they are too big, or would interfere with the circuit somehow.

I suspect that's pretty much it, and how that impacts cost.