CD transports - do they sound different and if so, why?

The title says it all..... if all transports do is take the data off the disc and send it the resulting ones and zeros in appropriate word length to a spdif output, how can different CD transports have any effect on the sound produced out of the DAC?

I can understand why different DACs will sound different due to the convertor and output stage, but not why transports should make a difference.

Can anyone shed any light on this? I understand about jitter - but if we had a means of eliminating jitter completely, would there still be reasons for differences between transports?

 

There is no way to eliminate jitter completely... :latte:

 

Besides jitter, a couple of other things are possible

1. RF Noise into the spdif output. I understand good cables fix this, but optical is pretty much immune from what I've read (could be wrong?)
2. If the transport can output an upsampled signal and the DAC can accept such a signal, then the sound can change (better or worse, it's your judgement)

Chord believed their DAC64 was transport independent until they came out with Blu. :duck:

However, stereophile measurements for the DAC64 still showed local word clock jitter at about 150ps (still pretty good).

 

Ah, it was about time this one came around again

Yes, but that has no bearing on whether it's transport dependant or not, it'll just be a feature of the DAC64 sound.

To be fair, I think the DAC64 is about as transport independent as it's possible to get without going to ridiculous and totally impractical lengths (eg, reading an entire CD into buffer memory before playing it out).

The Blu can change the sound by virtue of its upsampling options and I guess that's what Chord hope people will pay £4.2K for. Personally I'd be very surprised if there's much difference between the Blu and any other transport at 16bit/44.1kHz using a TOSlink connection.

Beyond jitter (which IMO the DAC64 does eliminate to the extent of making it insignificant), RF noise (which can be eliminated with an optical connection) and upsampling options (which I've tried and don't like) there's nothing else that can differentiate the sound of a transport. If there is, then I'd love to hear a scientific explanation for it.

Michael.

 

There are lots of ways.

Mostly it depends on the way in which the transport interacts with the DAC it is attached to, perhaps more than the transport itself.

For example, assuming SPDIF, the output signal itself is closer to an analogue RF signal than a purely digital one, at least in the ways in which it is sensitive. As a brief resume, and considering only the link betwen the two boxes:

• Accuracy of 75ohm output impedance and the way it interacts with the DACs SPDIF-input loading.
• Isolation of SPDIF output - whethere or not there is a pulse transformer to isolate the output from ground current interference.
• Ability of the SPDIF output to drive the interconnect (stray capacitance), and the interconnect itself.
• Inherent accuracy of the transport clock (which is rolled into the SPDIF signal to clock the DAC - BAD idea)
• How any and all the above affects the DACs ability to derive an accurate word clock etc.

- And that's overlooking any of the rest of the whole 2-box-soup! Of alternative connection methods Toslink suffers from higher jitter - it doesn't have to, but the economics of doing it properly mean that most don't - they use cheap optoisolator output with insufficient speed, powered from the internal digital 5v line with noise on it, same clocking problem as SPDIF. Better connection methods, like AES/EBU or I2S reduce some of the sensitivities, but unless there is an accurate, independant masterclock for conversion in the DAC there will always be some interdependance on the transport.

 

Afterthought:

Quite likely true, but there's a more elegant method, which is less complex than the DAC64s extensive PLL filtering: a small buffer memory in the DAC and a local master clock. The data is accurately clocked out for conversion locally, and the buffer is only picking up the difference between the clocks rates. Why more DACs don't do this I don't know - it should be completely independant of the data source or connection method. Probably a cost thing.

Then again, at this level the audible differences due to jitter etc. are swamped by the decisions made in the anti-alias filter and analogue stage design.

 

A well shielded cable would reduce the ingress of external RF I guess, but it won't help that generated within the transport itself...

 

felix,
the dac 64 does have a local buffer which it clocks data out of - hence the 2 or 4 second delay on start up. also meridian players read / re-read data into cascaded buffers for accuracy and de-jittering.

cheers


julian

 

Yes, but I believe the clock itself is derived from massively averaging the incoming signal, not a separate internal clock.

That's not to say it doesn't work very, very well

[Edit: I googled some more and found you are quite right - the chord does seem to have a local clock. Thanks Julian.]

 

Yes Julian, and just listen to how rivetting they are to listen to

Don't get too stressed about Jitter, its just one small part of the reason that most CD players sound unatural. If you think jitter really does sound bad, you should hear the effect of digital clipping on your CD's Like jitter only worse and on about 90% of popular CD releases.

the best transport has surely to be no transport?

 

This has been done to death
OK, I'm not going to mention the Blu transport, (for those wanting more boxes from the land of SMPS, then Jimmy Hughes review will leave you juicing your silkies, I wonder how long it took them to clean those 'stains' off his listening room ceiling
Intersting experiance I had last week, First time I've come across the Meitner Dac & Transport combo (EMMS labs?) (Philips 1000 multi format unit)
It's unique in the fact (I haven't found another one like it yet) it uses 3 AT&T glass optics (and these are the fabled orange versions too) to carry ALL the data between Transport and dac, output to dac, word bit sync clock return, the music data back.
Very much in the dcs vein but with more detail and bass, I asked the customer If he'd mind me slipping in a balanced AES digital lead, and doing an a/b (On the wadia's they reckon glass optic is THE method of Data transmission, now theroy says, only the contamination at the electrical <> optical ends, the rest should be zero interfence)
Personally never found this, even when we built dedicated psus just for the optical pathways, even upped the spec of the laser transducer??? Always prefered AES/EBU
Anyway, I was expecting a down turn in sound quality, humm, well it injected some life into and ceratinly gave the vocals more tangability and real feel?
I was some what surprised to say the least, could this be a medicore supply rail to the glass arrays? (judging by the build quality er. no), does the balanced have a quiter supply? dunno
If the are both working correctly, does this mean then may a little jitter (colour) may not be a bad thing after all
Just some sunny day musings

 

Ah - I see you found out yourself that the DAC64 does have it's own local clock which is not derived from the incoming clock signal in any way.

Michael.

 

The last time we did this topic, this happened.

Plenty in there to be going on with, although there were some issues still extant IIRC

 

merlin,
i was only pointing out a couple of manufacturers who use the scheme that felix described. technologically i respect all the design effort that goes into this way of doing things and i apprieciate that it seems to be 'logically' the 'best' way of doing things. at the end of the day though if it sounds kak then well it is kak - the debate about what is kak and what isn't is entirely subjective though. so in essence i agree that jitter is a small but important part of the overall sound of your system,
as for:

what do you mean? just have a dac sitting there with the open end of a fibre optic or digital cable pointing skyward listening to the music of the sphere's then?
cheers


julian

 

Ooohhhhh Bollox you've rumbled our star product for the Heathrow show, and its taken 2 years of development to get this far :MILD:

 

I had a look at the old thread from february... I think I'll hold off thoughts of changing transport for now....

thanks for the input.

 

No harm in going to a HiFi shop for a demo to hear for yourself. Well none bar the damage to your bank account anyway...

 

i think the bottom line comes down to the fact that nothing is ever perfect, so a dac is never going to be able to be perfectly immune to jitter or other effects from a transport (or digital cable / optical cable if you go for that sort of thing) so there will be subtle or not so subtle percieved differences between transports due to the interaction between the components used. when someone says - that's a warm sounding transport, i believe what they really mean is, that's a warm sounding transport with the dac / cable / system i was using. the thing to do is try as many things as you can within your system if at all possible.
not much help really.
cheers


julian

 

something glaringly obvious that hasn't been written yet.

A transport is actually quite simple with regard to the number of things inside, not individual bits but functional blocks.

The motor spins the disc, the laser reads it, outputs it to an rf amplifier, gets sent to a chip for the disc info, and has a servo, and possibly a decoder.

Digital is not my forte, but one thing I think is hugely significant is error correction, there is a mechanism to correct errors and scratches in discs, not sure how it works exactly, usually called 14-8 or something.

Dont' forget the transport control, this is done by servo chips, and they need to be top quality to control the laser and disc reading, if the whole spinning( motor, laser, tray, etc., ) is the football team, the servo is the manager ( bad analogy).

Then there is the transmission down to the dac.

I am not 100% convinced about jitter myself, I have a cd with 600 pico seconds, which is highish, and I like the sound a lot.

The dac 64 I had the pleasure of, and its not quite a musical to my lugs as a naim cd, more mechanical, (output stage, power supply? WM???), its still very good. The naim cdi probably had more jitter, so again, dont' take one element in isolation.

You takes your pick.

So the block of a transport are the mehcanical bit itself, laser, motor

then the servo which controls this,

then the chip that receives this, the decoder with error correction and other bits

then the display chip

and the transmission stage down to the dac.

simple really, all powered by the ubiquitous psu.(transformer, diodes,caps, voltage regulators)

I have come to the conclusion its futile to ask why something sounds better or worse, 1- its personal taste 2- I don't have a clue!!

 

There's a lot about this in the old thread if you'd like to have a look through it, with all the links you could want for further reading.