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NEC's mysterious digital filter and DAC chips

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thatsright

thatsright

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I have the Nak OMS-4/40 SM. It uses two 74-series ICs to split the data signal for the two PCM54s.

And yes: The PCM54/55/56 do indeed require:
"A Sample/Hold Amplifier (Deglitcher) is Required at the Digital-to-Analog Output for Both Left and Right Channels."
From:
The Nak seem to do this purely discretely (see schematic, p. 36).
That last comment, emboldened, might be wrong -- my bad. But introspect for yourselves and comment as necessary.
In the Nak OMS-4/40, there is a logic splitter after that mysterious 6352 DF. I think it splits the data line into L and R. One for each of the two PCM54s. The 6352 also has the necessary multiple data-line outputs that "Y-off" to both the two 74-based splitter and the two PCM54 dacs. [Actually, several decoders and DF chips from that era had multiple data lines, such as Sony's CX23035 https://vasiltech.narod.ru/files/CX23035.pdf ].
However, because the split was done before the PCM54's, there is no longer a need for s/h switching following the dacs.

Sony (CDP-c-505) used a dual PCM56's , but with a much simpler strategy. The '56 had the tradit three I2S pins . The CXD2551 DF splits out separate DATA L and DATA R on dedicated pins (so the DATA does not need to be split externally; however, WS, or LE, does need to be switched if two "mono" DACs follow] See attachments. Note also that the PCM56 no longer uses multiple data lines.
But '53-'56 all needed analog switching for stereo operation if only a single PCM53-56 was utilized.

BOTTOM LINE: For PCM53-56 series, if you split the DATA signal into separate L and R, using appropriate glue logic, you just need the std. I/V and LPF stages after the DACs.
Aside from the S/N advantages of using two DAC chips, I'm not sure which "sounds better"? Digital switching (splitting) of L and R before the two DACs or analog switching after the single DAC? Which is theoretically faster?

Screenshot at 2023-10-19 13-42-53.png


Below is a SM block diag. from a 1986 Sony CDP-65 that uses a single PCM54. Note the analog switching and S/H.
Screenshot at 2023-10-19 14-40-16.png
 
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restorer-john

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That Philips CDR765 [a cd recorder with analog inputs] uses two tda1305's, but one for the cd player (tradit. stereo analog output) , and the other from the analog input to adc to dac (the other 1305). This works like a cutting lathe DDL (digital delay line) or a 3-head cassette deck, so that you can listen to what you're recording "live".
It's nothing like a three head deck or a delay line. You cannot listen to the actual recording until after it's recorded and finalized.

The only reason there are two D/As in that machine is because the loader mechanism used (CDM 12.4) has the D/A onboard as a complete CD playback unit. The other D/A is simply for monitoring the converted digital input and set levels etc. Like a two head deck in record monitor mode.

It's just another typical bodge by Philips trying to play catch up with the Japanese at that point.
 
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OP
thatsright

thatsright

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From this front photo :https://audioaddictsforum.com/thread/1845/nec-903-monster-high-vintage
It seems that the NEC 903 have a "remote controled attenuator system" . I guess it's what you take for a R2R DAC .

I think that the four DACs are here :
cd-903(2).JPG

https://audio-heritage.jp/NEC/player/cd-903.html
[Note: about the metal bars above certain chipsets .... they could also be grounded to create a sort of Faraday cage.]

Yes, I did count the # of legs (pins) on the one side I can see in that photo and they seem to concur with:

[NEC CD-903 ---- 4 x PCM56P-K + NEC µPD6352ACA ---- KSS-123A]
...and:
... and:

HOWEVER, the eponymous mystery deepens here because of what the Dutch and daclist claim about the DF / DAC combo. As I noted earlier, the PCM56 and PCM53-55 are very different DACs. The PCM53-55 are 28-pin old-skool PARALLEL INPUT DACs that need 17 data lines for the 16-bit ladder network and switches (see: https://vasiltech.narod.ru/files/PCM54-55.pdf). No WS(LE) or BCK input -- the parallel data comes into this dac, no holds barred, for pure conversion; that's it !

The PCM56, is of the typical "I2S" 3-line SERIAL INPUT dac with DATA/WS/BCK. https://vasiltech.narod.ru/files/PCM56.pdf

So, I'm unclear about PCM56P + NEC µPD6352ACA, because the NEC µPD6352ACA seems to be designed for the older style (non-I2S) , as I noted earlier in ref. to the Nak OMS 4/40.
See below [from Nak OMS 4/40 SM].

Screenshot at 2023-10-19 23-16-25.png
Screenshot at 2023-10-19 23-15-55.png
Screenshot at 2023-10-19 23-15-22.png
 
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OP
thatsright

thatsright

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So, I'm unclear about PCM56P + NEC µPD6352ACA, because the NEC µPD6352ACA seems to be designed for the older style (non-I2S) , as I noted earlier in ref. to the Nak OMS 4/40.
See below [from Nak OMS 4/40 SM].
Well jeez-o-pete ... that Nak OMS-4/40 is a complicated beast! First, it's important to note that it uses a pair of PCM54 dacs, which have parallel data input lines (not like PCM56 and later, that use the typical serial input lines).
Anyway, if you look closely at the OMS-4/40 sm schematic, you'll note that 6532 IC parallel-outputs .And only one channel (L channel PCM54, the uppermost of the two) go thru the messy DF/F IC305/306 messy 74-hc glue logic first. This splits the "multiplexed" (not sure how else to define) stereo signal into separate L and R for two '54 DAC chips.
I can't find any photos of interior but the repair video below can be paused for some glimpses. The 6532 is located on the main horiz. PCB, while the DF/F and DACs get their own plug-in modular cards (see SM). I'm not sure how phase accurate the data stays going thru all this mess? Cassette-deck experts shooting for digital. Ugh! In any case, the Nak players were $$, but got good reviews.
 

restorer-john

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@thatsright What PDF reader are you using in LM? I'm looking for a decent one is that one good with large docs/searching etc?
 

audio_tony

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@thatsright What PDF reader are you using in LM? I'm looking for a decent one is that one good with large docs/searching etc?
@restorer-john I've been using Sumatra PDF reader for several years. It's free and seems to handle large service manual pretty well.

There is a portable version as well, if you don't like installing stuff.

 
OP
thatsright

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Any simple PDF reader will work. I even use older Windows Acrobat in WINE if Xreader (native Linux) runs into problems. The main issue is that many older SMs are not OCR'd, or they were so poorly scanned that if one does go thru hassle of OCRing them (I use Windows Phantom PDF in WINE for LM), the encoding is poor.
 
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