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Old 27th December 2012, 02:19 AM   #1
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Default Sony "staggered" twin TDA1541

A myth seems to have arisen concerning the operation of the two TDA1541 DACs in Sony CD players such as the CDP337ESD. Sony claims 8X oversampling from "staggered" operation. The myth is that they are really just connected in parallel.

In fact, data inputs and current outputs are connected in parallel. However, the digital filter is set for 8X oversampling: double the processing rate of each DAC. One DAC WS input is clocked by the WS output of the filter, and the other by the not-WS, so they do indeed alternate, each processing 4X, resulting in 8X output.

Parallel TDA1541/A has a bad reputation that has been extended, unfairly, to the Sony implementation, which if nothing else does indubitably have the benefit of the higher rate.

I've never quite grasped why monotonicity is important, but it occurs to me that alternating monotonic dacs may not combine to give a monotonic result. That is, an incremental rise in the input could lead to a fall in the output, or vice versa.

Anyway, just to put the record straight. The historic and illustrious name of Sony needs all the help it can get.
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Old 27th December 2012, 02:53 AM   #2
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Just had a scan through my CDP-507ESD service manual, and you do seem to be correct about the dac arrangement. Whether it's advantageous versus straight parallel remains to be determined. I've just acquired a 507ESD, which I consider fundamentally better than the far more expensive(even now) 707ESD, and plan to see just how good I can make it sound sometime in the next few months. It's interesting that the 507ESD uses half of each TDA1541A-R1 for one channel, and the other half for the other, rather than the more common use of a whole dual dac per channel. Makes for simpler l/r clocking, I suppose, and easier layout.
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Old 27th December 2012, 04:25 AM   #3
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Quote:
Originally Posted by stephensank View Post
It's interesting that the 507ESD uses half of each TDA1541A-R1 for one channel, and the other half for the other, rather than the more common use of a whole dual dac per channel.
Are you sure? My interpretation is that both DACs get all 8 oversamples for both channels, and each ignores alternating L/R pairs. That is, they both operate normally at 4X.

But there is an apparent anomoly...8X output steps each with the period of 4X takes twice as long as real time.

So the outputs overlap like bricks, and consequently the output level from the IV should be double, just as for normal parallel DACs. This, it occurs to me, is why it looked so bad when tested with RMAA...it was overloading the input to my rudimentary soundcard. Now I've made a buffer/headphone amp, so I can test it again with the necessary attenuation.

As for the sound, it's delicious. How much that comes from pride of ownership, I can never tell. I got worried about mechanical noise from the irreplaceable KSS190A and now I'm using an Arcam Alpha. When I swap back I'll be in a better position to compare. I guess the biggest difference will be between the analogue filters.

Also using, with my headphone amp, a delightfully simple little CDPM75...single TDA1541 and just one dual opamp serving both channels...sounds good too.
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Old 27th December 2012, 05:48 PM   #4
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I was merely saying that each dac chip is used r channel to r chan, left chan to left chan, not commenting on the sampling in that regard. Most parallel dac implementations use a dual dac's left & right channels in parallel with each other for one channel, then same for other.
As to KSS190 acoustic noise, no cause for concern, IMO, as it is pretty darn certainly just the focus gain and/or tracking gain being turned up higher than needed. All you need to do is play the cd/cdr the player takes longest to start up(if there is a difference), then turn each gain pot down(which ever way reduces noise, usually ccw) to the point where play starts failing, then turn pot back up about 20 degrees of rotation. Then verify it has no trouble starting up any other discs, or skipping tracks, etc. Other thing to cause excess noise is if focus bias(focus offset) is off. Usually the best set point corresponds to minimum noise point, but not always, and it really should not be adjusted without an oscilloscope, using which one shoots for maximum RF pattern amplitude & clarity.
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Old 28th December 2012, 12:50 AM   #5
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Ah. I didn't know what most parallel implementations do. I've seen plenty of TDA1543 stacked in parallel, where all the Rs do the R, and the Ls the L. AFAICS for the Sony arrangement that's the only way to achieve the 8X, which is why I mentioned it.

As for my KSS190A, I did all the adjustments by the book. Had 0.8V RF only. Renewed all relevant caps and readjusted, to get barely 1V, just within spec. Finally adjusted FO by ear, but there's still an episodic tickytacky noise from the mech. Not intrusive like it was, which is why I'm going to reinstall it.

Search and play for all disks has never been a problem...almost instant thanks to the linear sled motor. The eye pattern is sharp and clear, but there's spikes on the crests and, during the noisy spells, its DC offset jumps in time with the rotation-speed tick. I guess it's the focus reaching the end of its travel. No idea what the spikes are about. Philips are fuzzy, Sony are spiky, I thought.

One thing I'm about to check is whether the CD is allowed to tilt beyond allowable limits. The tilt could vary with the jogging of the sled, explaining the episodic nature of the noise.

I'm hoping someone else will rebuild one of these heads before I have to.
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Old 28th December 2012, 02:21 AM   #6
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Quote:
Originally Posted by PlasticIsGood View Post
In fact, data inputs and current outputs are connected in parallel. However, the digital filter is set for 8X oversampling: double the processing rate of each DAC. One DAC WS input is clocked by the WS output of the filter, and the other by the not-WS, so they do indeed alternate, each processing 4X, resulting in 8X output.
The TDA1541A has two possible input modes, the I2S mode won't accept more than 4X OS as there's a limit on the bit clock speed (6.4MHz). The mode which works faster has separate L and R data inputs. Check to see if pin4 has any activity - if so then its using the simultaneous data mode.

If the I2S WS pin is merely inverted then there will be smearing - adding together of adjacent samples - because the update of the output is always done on the clock cycle subsequent to the falling edge of WS. To handle this correctly without distorting the output (incurring roll-off) would require a delay line.
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Old 28th December 2012, 04:52 AM   #7
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The output from the filter chip is set to 8X, I2S mode. I assumed the TDA1541 are each 4X, I2S. They alternate and overlap to achieve the 8X. There's no delay. I assume, but perhaps should check, that there is none between the WS and not-WS outputs of the filter, but it doesn't seem likely.

Checking the TDA1541 datasheet, it's set for I2S it seems, with pin 4 tied to pin 2 (BCK) and pin 27 held high.

What I'm uncertain about is exactly how the DACs operate their input registers and latches. The data, and presumably the WS signal, arrive and disappear at twice the rate expected by each DAC. Unexpected signals are presumably ignored.

Smearing would be quite an oversight on Sony's part. I think I see what you mean, but don't see how it applies here. The outputs are completely overlapped. With two 4X chips sharing 8X data, there's no time for them to be anything else.
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Old 28th December 2012, 05:28 AM   #8
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It certainly applies to all I2S DACs and it bit me when I tried this trick of swapping channels, using TDA1387s. You have updates going on at 8X but the DAC with the inverted WS does not just have the channels swapped over - because WS is used to time the updates and frame the data.

I2S is sent left first, then right. When you invert the WS you don't suddenly get right sent first, rather you end up treating the right channel of the previous sample as your new (current) left channel. This gets paired with the current left, re-labelled as current right. Hence the stereo pair isn't a true pair any longer.

An easy oversight to make, but it remains an oversight nevertheless.
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Old 28th December 2012, 07:00 AM   #9
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There is nothing in the CDP-507ESD schematic to suggest that the WS for the second dac is merely inverted. What you're describing would not have been missed by Sony's design staff, I'm sure. And no channel swapping is going on. Each 1541's left channel is used for the left channel, and each right channel is used for the right channel. I believe the WS signal is simply "demultiplexed", one pulse fed to one dac chip, the next pulse sent to the other. Each dac thus simply handles half of the samples. What is not clear to me is if the output currents sum or not. Seems to me that the output current from the two dacs should stay the same as for one dac, since they are alternating, like two people hammering a nail, so you get twice as many hits, not twice the force. I will be able to confirm or debunk all of the above once I have time to upgrade the 507ESD.
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Old 28th December 2012, 07:04 AM   #10
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Thanks Stephen, that's helpful - but the interleaving of two 4X OS DACs does not an 8X OS DAC make.
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