Asyncronous reclocking of transport SPDIF output

Dr.H

Member
2002-04-08 6:41 pm
Hi all,

Based on some discussons here, I recently tried to reclock the SPDIF output of my Rotel 855 cd player (used as a transport) using 74HC74 fed from a 66MHz clock (asynchronous)

In stock form, the Rotel uses a 74HCT08 buffer and I placed the 74hc74 between the oversampling chip and the 74HCT08.

I have noticed very small pops occasionally coming through when music is playing.

Couple of questions:
1. Is the 74HC74 in the right place, i.e. should it be AFTER the '08?
2. Transformers on digital out appear attractive because of the comon mode impedance provided. Could I place a transformer after the '08 or should I try '74 after '08, with '74 feeding the output transformer?

Thanks for any help!
Ryan
 

kevinkr

Administrator
Paid Member
That is what I suspected, although the differences are very small compared to the period of the actual waveform. I am not sure that a high bandwidth differential driver would be much better in this regard due to delays. I think it is questionable whether most cmos logic would drive AES/EBU or even spdif well single ended (roughly 220 ohms total with series and back termination resistors on AES/EBU) through a transformer with a ratio of 1.4:1 which is why the Lambda uses differential drive with a ratio of about 3.3:1 or 4:1 .
Perhaps paralleling a couple of gates for higher output current would work somewhat better..

When the lambda worked it sounded good, few transports at or above the price surpassed it let alone equalled it.

Kevin

Edits to fix embarrassing typos.. LOL
 

kevinkr

Administrator
Paid Member
My question had absolutely nothing to do with the asynchronous clocking question, but rather Jocko's assertion that driving a spdif output transformer with the Q and Q_bar outputs of a flip-flop was a bad idea. I understand in theory why this is not the best thing to do, but NO one so far has furnished evidence of a superior way of accomplishing this task. Every approach has its pitfalls.

The Lambda Drive I own commits just this faux pas, and demonstrably sounds better than almost anything I have yet heard except for some very expensive teac/esoteric drives.

Kevin
 

rfbrw

Member
2001-10-26 11:51 pm
.
kevinkr said:
My question had absolutely nothing to do with the asynchronous clocking question, but rather Jocko's assertion that driving a spdif output transformer with the Q and Q_bar outputs of a flip-flop was a bad idea. I understand in theory why this is not the best thing to do, but NO one so far has furnished evidence of a superior way of accomplishing this task. Every approach has its pitfalls.

The Lambda Drive I own commits just this faux pas, and demonstrably sounds better than almost anything I have yet heard except for some very expensive teac/esoteric drives.

Kevin

Perhaps a re-reading of the title of this thread, not to mention the first post, is in order.
 
Hi RFBW,
Yes I know what the title of the first post was and the initial subject of the thread - I was an early participant, but as I stated earlier my question was aimed at a specific assertion that Jocko made in regards to generating the spdif output - which was admittedly an aside to the general topic of the thread, but appeared in this thread. My question was seeking clarification of that assertion only, and I intended no offense in my response.

I was actually hoping Jocko would reply in person, as you may note in my post the question was directed solely at him. I can think of several reasons why this might not be the optimum approach, but as he is regarded as something of a guru on such issues I really wanted to hear his response.

Also as previously noted many theta and all PS Audio transports drive their aes/ebu and spdif transformers directly off of the Q and Q_bar flipflop outputs, and sound good doing it.

:) :)

Kevin
 
I have answered that question the other day on another forum. Rather than advertise the fact, I let the others say the same thing. But if you are enterprising you may be able to find the answer.

Anyway.............as rfbrw points out, that will be the least of your problems. But go ahead............try it anyway. You'll find out the hard way. Two bad ideas combined can make for some really rotten sound.

(But if you put Wadia on the front panel, no one will know! Amazing how many people can hear dreck, and think it is hot snot just because it costs $8K.)

Jocko
 
I've absolutely no doubt at all that asynchronously clocking spdif is the wrong way to go, I was just curious about the flip flop issue. Jocko, I will look for your posting on the other thread.

I can't talk to the Wadia issue, the last one I heard was at least 10 yrs ago, and for whatever reason, price and label on the front panel aside, I didn't like it.

Kevin
 
Extreme_Boky said:
Hi Dr. H,

Check this (excellent!) link. It explains a lot and will help you make your own decision. Try making it synchronous and see if the clicks disappear.
http://www.lessloss.com/types.html

Cheers,
Extreme_Boky

The clicks are probably due to the metastability effect. Try my ASR with two flip-flops in series and that problem will be solved. :idea:
 

Dr.H

Member
2002-04-08 6:41 pm
HI Elso,

Thanks, I meant to include that option as well. I noticed on Pedja Rogic's website that he uses the 74 in this way, i.e. d1 clocked to q1 which is input to d2 and clocked to q2, both lcocking processes using the same (asynchrocnous) clock.

Will report back
 
I have built Pedja's DAC + reclocker.

I listened to basic DAC for 3-4 weeks and played with different capacitors and grounding topology. Once I was very happy with the basic DAC, I added Pedja's reclocker with EXCELENT result.
I will post my findings about different stages of my tuning and impressions about this DAC in general.

Just to add, this DAC & reclocker together with Audiomeca Memphisto II transport produced truly amazing results. I don't think there's any "digital" combination / stand-alone player out there that approaches analog closer than this combination.

Regards,
Extreme_Boky
 

rfbrw

Member
2001-10-26 11:51 pm
.
Extreme_Boky said:
I have built Pedja's DAC + reclocker.

I listened to basic DAC for 3-4 weeks and played with different capacitors and grounding topology. Once I was very happy with the basic DAC, I added Pedja's reclocker with EXCELENT result.
I will post my findings about different stages of my tuning and impressions about this DAC in general.

Just to add, this DAC & reclocker together with Audiomeca Memphisto II transport produced truly amazing results. I don't think there's any "digital" combination / stand-alone player out there that approaches analog closer than this combination.

Regards,
Extreme_Boky


Fair enough, but asynchronously reclocking the spdif datastream at transmission is not a good idea.