ChanServ changed the topic of ##yamahasynths to: Channel dedicated to questions and discussion of Yamaha FM Synthesizer internals and corresponding REing. Discussion of synthesis methods similar to the Yamaha line of chips, Sound Blasters + clones, PCM chips like RF5C68, and CD theory of operation are also on-topic. Channel logs: https://freenode.irclog.whitequark.org/~h~yamahasynths
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<l_oliveira>
@cr1901_modern regarding the CDTV thing on twitter, I had a *bizarre* fault on a MEGA-CD console which was the sanyo LC8951 IC where it had a internal fault which caused CD-ROM sectors to be retrieved with corruption on the data
<l_oliveira>
I wonder if that Mitsubishi decoder could have a similar issue in it. First time I ever saw a CD-ROM decoder solution from Mitsubishi
<l_oliveira>
replacing the LC8951 chip made the console resume normal operation
<cr1901_modern>
I really need a chunk of time where I'm not on someone else's clock to look into this stuff soon. I'm confused over the block diagram because I don't think I understand EFM layout properly
<l_oliveira>
I originally suspected the buffer ram connected to it but the ram was perfectly fine
<l_oliveira>
Since it is an AMIGA, would be sensible try to make it run some sort of test software and look what it is reading from the discs
<cr1901_modern>
I find it odd that the disc is failing so bad that error correction is failing to correct everything, but the disc still puts out something legible for a bit
<l_oliveira>
Actually what is important about the eye pattern there is the signal being stable and uniform.
<l_oliveira>
If you get stable signal off a audio disc it will not skip even with relatively big scratches as there's enough data for the reed solomon to correct the bits
<cr1901_modern>
Also, the audio is interleaved in such a way that each 16-bits are actually a single bit of audio for 16 samples
<cr1901_modern>
IIRC
<l_oliveira>
On the case of the data, theres another layer of error correction on top which is inside of the cd decoder chip, if the chip has a fault internally that might fail
<l_oliveira>
so you get a situation where audio discs plays perfectly but data reading is unstable/random
<cr1901_modern>
I didn't know analog scopes could do eye patterns, tbh. Relying on phopshor persistence?
<l_oliveira>
A 20mhz scope is enough for that
<l_oliveira>
indeed
<cr1901_modern>
If the "eyes" weren't evently distributed, that would indicate high jitter, correct?
<l_oliveira>
if you have jitter from the mechanism it will wooble vertically (bent turntable) or horizontally (failing spindle motor causing speed oscillations)
<l_oliveira>
if you have problems with the optical pickup electronics you might have "dirt" inside the waves that form the pattern
<l_oliveira>
the signal there look absolutely gorgeous, which makes sense since it's a caddy load unit
<l_oliveira>
the turntable must be very solid
<cr1901_modern>
>wooble vertically
<cr1901_modern>
The slicer circuit (which TIL! Clever) would take care of this and force the voltage back down to the correct reference inside the chip
<l_oliveira>
well that signal is sent to the DSP
<cr1901_modern>
ahhh
<l_oliveira>
the CD-ROM decoder typically takes a tap from the I2S stream which goes to the DAC for the CD-DA output
<l_oliveira>
when playing music it goes to the DAC and when decoding data it goes to the CD-ROM decoder
<cr1901_modern>
What does "takes a tap" mean? Wouldn't it feed the DAC values, not receive them?
<l_oliveira>
like a multiplex
<l_oliveira>
both chips receive it at the same time but only one of them is active at a given time
<l_oliveira>
DAC gets muted when the drive is reading data
<l_oliveira>
decoder does nothing when playing audio
<cr1901_modern>
>both chips DSP -> MUX -> DAC
<cr1901_modern>
|-> CD-ROM Decoder?
<l_oliveira>
DSP outputs stream, both chips get it at the same time
<l_oliveira>
format can be I2S or right-justified 16bit stream
<cr1901_modern>
Oh, so a CD-ROM decoder accepts I2S as input
<l_oliveira>
depends
<l_oliveira>
LC8951 is for 16bit justified, LC89515 is for I2S
<l_oliveira>
newer stuff uses LC89515 on videogames (perhaps because I2S prevailed?)