Digital audio signals are only zero's and one's and can't go wrong, now can they?
Well they can, big time…. Well, in digital signals it's more 'small time', leading
to big analogue degradation. So what can we do to improve the digital signal?
Lets start with a confession: I deliberately used a misleading name for this video.
Although I will discuss improving digital audio signals later on, the most important thing to do with
digital audio signals is to handle with care so they don't get damaged in the first place.
In "Connecting your DAC #2: how digital can go wrong" I explained why digital signals
can be distorted in a way that it has consequences for the analogue signal after digital to analogue
conversion. See the link in the show notes. In short, the quasi square wave - which by
the way is analogue by nature - can and will be distorted when sent from one device to
another, whatever you do. What you would like, is a perfect square wave that have zero rise
and fall times. The problem is that this can only be achieved if the signal path has unlimited
bandwidth. Since there is no such thing as unlimited bandwidth, the square wave will
always be distorted to some degree. This is not really a problem as long as it is a stable,
limited and constant distortion. As I have shown in the earlier mentioned video, things
can become a problem if the distortion of the square wave is too serious. Of course,
error correction will be able to correct errors but this takes time and in digital audio time
is very critical. Things get even worse if the square wave varies in time. Again, see
the aforementioned video. The question is how to prevent this all.
As with everything audio, the chain is as strong as the weakest link. This means that
every link in an audio system must be just as 'strong' as the others. So you have
to start with the digital source, whether it a cd-player, streamer or computer. The
digital output - whether being SPDIF, optical, AES/EBU, I²S, USB or any other standard,
needs to be as close to the standard used. For instance for SPDIF the output needs to
be 75Ω and point 5 to 1 volts. Often standard integrated circuits are used. I suppose these
are perfectly within spec, but that might be naive. Then this circuit should not be
under the influence of other strong signals like clock signals or for instance magnetic
stray of servo power lines. Also the power that feeds this circuit needs to be very constant
to guarantee a steady square wave. And then the output connector might cause a deviation
from the standard. Let's take SPDIF as an example: this usually is present in the shape
of a the RCA connector, which is not designed for 75Ω so here is another potential problem.
The sad thing is that a BNC connector will cost hardly more and are available in 75Ω.
There by the way also is a 50Ω variant, something to keep in mind when you want to use BNC.
The next step is the cable. This will have a connector on both ends and usually - there
again - are RCA's. Then the cable should be 75Ω too and preferably over a wide frequency
spectrum to keep the distortion of the square wave due to bandwidth limitations as low as
possible. Shielding is also of great importance, strong cell phone signals, wi-fi signals and
other high frequency pollution must be kept from the digital signal. On the receiving
side - usually the DAC - we will find an RCA input again, the print design and the receiver
chip, all potential places where things can go wrong. Now what can you do about this?
Let's take this step by step.
Whatever digital source you choose, the digital output quality is a property of that device
and in most cases can not be changed. If you're handy you could sometimes replace the RCA
connector for a BNC. Only if the basic quality is already rather good, this might give some
improvement. Many networked audio adaptors come with cheap external power supplies. Replacing
these for an audio grade power supply can give clear sound improvements. If tweeting
is your thing, the web might suggest all kinds of modifications. Be careful though, often
these mods shift properties out of balance. That might work for a given situation to compensate
for shortcomings of the other equipment. But it might have the inverse effect in your situation.
Also take in consideration that modifying your equipment might invalid your warranty.
As I have mentioned before, a digital audio signal is sent over a cable using analogue
square waves. So the correct impedance over a wide spectrum is of importance, as is the
shielding and the mechanical construction of the cable since poor mechanics can cause
microphony, the generation of small voltages due to variation of the distance between conductors.
A cheap solution for SPDIF - if you can use BNC connectors - are the video leads for professional
video applications. A few euro's a piece and not bad at all. But a good quality cable
made for digital audio is even better. That not only goes for SPDIF but for all digital
audio cables, including USB.
Whether you use a separate DAC or a DAC integrated in an amp or other device, the same will apply
as for the digital source: most things can't be changed so you better choose a device that
performs well. This of course is related to the price. But is hardly ever related to a
given part. Most popular on the web currently is the DAC chip. People carefully compare
the specs and think that a higher spec chip will make a device sound better. It might,
but it's no guarantee. Again, the quality of the clock oscillator, the distance between
the oscillator and the DAC chip on the PCB, the receiver chip used and its place in the
PCB, the power supply, HF radiation near the DAC board, it all has its influence. So don't
think that a HAT board for the Raspberry Pi, containing an ESS 9018 DAC chip will sound
the same as a far more expensive USB DAC using the same DAC chip. So choose wisely by comparing
reviews and - if you have mastered the art of interpreting forums - that might be a good
spot for information too. Of course the power supply, when external, can be replaced by
an audio grade one, but that's about all you can do as normal consumer.
A reclocker is a device that takes a digital signal, analyses it and tries to reshape the
square waves or generates completely new ones if it is a better design. This will improve
the sound quality when done right and when the input signal was poor of quality. But
it often make me think of youngsters buying a very old Volkswagen Golf for a few hundred
quit and over time stick plastic body parts and a spoiler on it. If they had saved up
somewhat longer and spent the same total amount on a better car, they would have had a far
better car. Cheap reclockers are like spoilers: window dressing. The term cheap reclocker
is a contradictio in terminus, terms that contradict each other. A good reclocker must
use high spec clock oscilators and those are expensive. They should be mounted on a very
well designed PCB together with all other high quality components and should be fed
from a very clean power supply. So reclockers are nonsense? No, but cheap ones often are.
If you choose the best digital source you can, chances are that a reclocker that will
improve the sound quality is far beyond what you will afford yourself. The same goes for
the DAC: good designs already use schemes to reduce jitter. Some DAC chips even has
jitter reduction integrated. Only if you have all the other equipment in order and you have
money to spend, you might consider a reclocker. I can think of only one other situation where
you might use one: if you for some reason want to keep using the equipment present and
do want to improve the sound quality.
Let me be clear: I have no objection to reclockers but they should only be of very good quality
to be useful and often the same or better result can be obtained by spending that money
on the source or the DAC. A viewer that wrote me he would start with buying a networked
audio adaptor and a new DAC and then would save for a reclocker, triggered this video.
I don't blame him, he just fel victim to the commercial mumbo jumbo of some vendors.
Be smart, be informed. My respected colleagues and I try to weapon you. So subscribe to this
channel or my newsletter or follow me on Twitter, Facebook or Google+. See the show notes for
the links. If you have a question, post it below this video but please don't ask me
for buying advise. See my About Questions video to find out why. If you liked this video,
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Just one dollar a month will do. The link is in the show notes. And don't forget to
tell your friends on the web about this channel. I am Hans Beekhuyzen, thank you for watching
and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music.
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