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Reclockers - what EXACTLY do they do?

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Could someone please provide me with an exact description of what a reclocker does to the data stream. I know all the generic stuff about removing jitter etc but I’m interested in exactly what is happening to the data stream that is processed by the reclocker.  I would like to understand a little better why reclocking a data stream ahead of an asynchronous DAC has a positive impact on SQ or why an ultra smooth Linear Power Supply transforms the reclocker. Again I’m not looking for a generic high-level description, I’m looking for the nitty gritty step-wise process of how the incoming data stream is buffered, processed and retransmitted. My deep appreciation to anyone who takes the time to explain the actual nuts and bolts of digital reclocking.

Cheers!

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There is no single definition of a reclocker. What devices termed thus do depends on the type of signal they operate on. Since you mention asynchronous DACs, USB would appear to be relevant. The USB reclockers on the market are hubs, plain and simple. The USB 2.0 hub spec includes this figure and text:
 

Quote

 

image.png.5fea69b82aeb8b3b3bdbdc522c5534d2.png

 

High-speed packet repeaters must reclock the packets in both directions. Reclocking means that the repeater extracts the data from the received stream and retransmits the stream using its own local clock. This is necessary in order to keep the jitter seen at a receiver within acceptable limits (see Chapter 7 for definition and limits on jitter).

[...]

The half-depth of the elasticity buffer in the repeater must be at least 12 bits.

 

The total latency of a packet through a repeater must be less than 36 bit times. This includes the latency through the elasticity buffer.

 

The elasticity buffer is used to handle the difference in frequency between the receive clock and the local clock and works as follows. The elasticity buffer is primed (filled with at least 12 bits) by the receive clock before the data is clocked out of it by the transmit clock. If the transmit clock is faster than the receive clock, the buffer will get emptied more quickly than it gets filled. If the transmit clock is slower, the buffer will get emptied slower than it gets filled. If the half-depth of the buffer is chosen to be equal to the maximum difference in clock rate over the length of a packet, bits will not be lost or added to the packet. The half-depth is calculated as follows.

 

The clock tolerance allowed is 500 ppm. This takes into account the effect of voltage, temperature, aging, etc. So the received clock and the local clock could be different by 1000 ppm. The longest packet has a data payload of 1 Kbytes. The maximum length of a packet is computed by adding the length of all the fields and assuming maximum bit-stuffing. This maximum length is 9644 bits (9624 bits of packet + 20 bits of EOP dribble). This means that when the repeater is clocking out a packet with its local clock, it could get ahead of or fall behind the receive clock by 9.644 bits (1000 ppm*9644). This calculation yields 10 bits. The half-depth of the elasticity buffer in the repeater must be at least 12 bits to provide system timing margin.

 

 

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44 minutes ago, mansr said:

There is no single definition of a reclocker. What devices termed thus do depends on the type of signal they operate on. Since you mention asynchronous DACs, USB would appear to be relevant. The USB reclockers on the market are hubs, plain and simple. The USB 2.0 hub spec includes this figure and text:
 

 

 

Not sure I am familiar with reclockers. What I would hear if it will be added in comparison to no-reclocker set?

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15 hours ago, mansr said:

With an asynchronous DAC, probably no difference at all.

 

Thank you. And what improvement one could hear with "correct" DAC?

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4 hours ago, AnotherSpin said:

Thank you. And what improvement one could hear with "correct" DAC?

I'm not sure I understand the question. A well designed DAC, whether asynchronous (e.g. USB) or synchronous (e.g. S/PDIF), will be just as good as a reclocking device at rejecting jitter in the incoming signal. A poor synchronous DAC may benefit from a reclocker if the source has excessive jitter.

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9 hours ago, AnotherSpin said:

 

Thank you. And what improvement one could hear with "correct" DAC?

 

There's nothing "incorrect" about an asynchronous DAC. In this context, "asynchronous" does not mean "out of sync." Rather, it means that the DAC does not depend on the source for timing/clocking, which makes the DAC more robust and jitter-resistant.

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7 hours ago, mansr said:

I'm not sure I understand the question. A well designed DAC, whether asynchronous (e.g. USB) or synchronous (e.g. S/PDIF), will be just as good as a reclocking device at rejecting jitter in the incoming signal. A poor synchronous DAC may benefit from a reclocker if the source has excessive jitter.

 

 

 

And now we have to ask.... how many poor synchronous DACs are there?

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1 minute ago, Ralf11 said:

And now we have to ask.... how many poor synchronous DACs are there?

I saw one once in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying Beware of the Leopard.

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3 hours ago, tmtomh said:

 

There's nothing "incorrect" about an asynchronous DAC. In this context, "asynchronous" does not mean "out of sync." Rather, it means that the DAC does not depend on the source for timing/clocking, which makes the DAC more robust and jitter-resistant.

Correct in a sense it may benefit from re-clocker. If such benefit is possible, what one may hear? What exactly may happen with SQ?

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Just now, AnotherSpin said:

Correct in a sense it may benefit from re-clocker. If such benefit is possible, what one may hear? What exactly may happen with SQ?

If a DAC benefits from an external reclocker, it is by definition flawed to some degree.

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6 minutes ago, mansr said:

If a DAC benefits from an external reclocker, it is by definition flawed to some degree.

 

Ok, I see. Reclocker is a completely useless piece of gear which has no effect on SQ if one has decent DAC?

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6 minutes ago, AnotherSpin said:

 

Ok, I see. Reclocker is a completely useless piece of gear which has no effect on SQ if one has decent DAC?

 

Yes, that is what mansr, and I, and many others here, would say.

 

Not everyone agrees, though.

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4 minutes ago, tmtomh said:

 

Yes, that is what mansr, and I, and many others here, would say.

 

Not everyone agrees, though.

 

Great! Hope those who have another opinion will voice it here as well.

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On 3/10/2019 at 5:56 AM, mansr said:

There is no single definition of a reclocker. What devices termed thus do depends on the type of signal they operate on. Since you mention asynchronous DACs, USB would appear to be relevant. The USB reclockers on the market are hubs, plain and simple. The USB 2.0 hub spec includes this figure and text:
 

 

Informative, thank you.

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