"Say you have a movie camera designed to take 100 picture frames per second, thus a frame every .01 second. You are pointing the camera at an ball moving from left to right at a constant speed, and you take a whole second of that motion, that is 100 picture frames. When you “play it back” with a movie projector that runs a picture each .01 second, everything would “be fine”, as intended.
But let us say that the camera is very “unsteady”. It takes a picture, then it waits .05 seconds, then it takes 5 frames within say .001sec, then a couple of frames at .01 second…. When you play it back, the projector “does not know” what happened at the camera, and the ball may like it is slowing at mid air for a brief time, then it zooms real fast some distance…. That is distortion due to timing error – time jitter. Note that if the camera was OK but the projector had jitter, that also would be a problem. So jitter counts at 2 places – at the camera (which is analogous to the AD), and at the projector (which is analogous to the DA). Jitter is important at the converters.
In one sense, jitter at the AD is more important, because once it takes place; it is in the signal forever. One can replace a bad DA with a good one, and that will eliminate the jitter issue of a poor DA, but what the AD does can not be undone.
Analogies can be misleading. In the case of movies, with enough still frames per second, the eye makes it looks like continues motion. In the case of conversion, it is the analog circuitry that takes the samples and makes them into a continues wave. But I chose the analogy of video, because audio and video (as well as many other applications) are fundamentally based on equal and precise time intervals. The time between each adjacent sample should be exactly the same, and if it is not, there is jitter, which will distort the outcome.
I said that jitter is important at the conversion. What about jitter in transferring say data from AD to a computer hard drive? The answer – it is not important, because we are just moving data from one place to another. You can move one frame every second, very slow indeed, or move a million frames a second, very fast. You can move half the data now, wait a while then move the rest of it… It does not matter, because you are not viewing it. But one you play it back you need the timing to be clocked precisely.
But some manufactures and sellers of clocks wanted to sell clocks, so they decided to convince the world that you need to clock everything. And with enough advertizing money, they where pretty successful doing just that. There are times when you need to use external clock box – when you want to have a lot of gear (AD channels) work together. But as long as you do not need external clock and you can use internal, use internal. It is not only cheaper, it is better!
What you need is the “best clock circuit you can make” that is very steady to be very near your AD circuit – short connection, good grounding… That is internal clock.
Say you take the same “best clock circuit you can make” and put it in another chassis. Will that be better? Not, it will be worse. You now have to deal with 2 chassis thus grounding issues. You have a cable that can pick interference, you have a cable termination imperfection, a cable driver, a cable receiver, and I did not even start… By the time your clock arrives from the clock box, it has so much jitter that it requires some “jitter cleaning circuitry” – typically a PLL circuit…. I pride myself for making very fine external clock circuitry, but no way can I make the external clock circuitry be as good as internal. Almost as good, yes, but never as good.
However, the clock BS’ers are still arguing that their external clock will improve the sound. There is some claim of a “proprietary clock signal” that will make things better. That is a crock if there ever was one! The clock box to the AD connection is a ONE WAY street. The clock “DOES NOT KNOW” what the AD is doing. What kind of a clock box signal is going to improve ALL the following an Ad's:
1. AD with a lot of jitter induce by 60Hz power line
2. AD with little jitter induced by 60Hz power line
3. AD with jitter induce from digital circuit noise
4. AD with jitter due to nearby radio transmitter
5. AD with jitter due to nearby power tools
6. AD with jitter induced from the digital audio data
7. AD with almost perfect timing
8. AD that is powered off…
This is analogous to a doctor that can cure all illness, doing so without any information about the patient…
One of the main offending marketing BS guys said that you can take a tone and have it sound better with jitter. You can alter a fixed tone with jitter, and can argue that you like it, or that you do not like it. But the alteration has to be deliberate for a specific constant tone (including fixed amplitude). You change the tone and the distortion changes... Jitter distortions are very complicated, and they are an INTERACTION between the clock timing AND THE MUSIC. The last I heard, music is not a constant fixed tone
The simplest of jitter (random jitter) will increase your noise floor. More complex jitter makes for all sorts of undesired at frequencies that depend on the music, but at frequencies that are not musical harmonics, thus sound bad…. I first stated that internal clock is best a few years ago, and had to deal with a lot of attacks on a forum I was moderating. I insisted that the technical folks come in, instead of the marketing types, and sure enough, the technical types backed off after a short “fight” because they had no leg to stand on. A couple of years later, Digidesign wrote a paper about clocks, and they second me by saying that internal clocks are the best (when you can use internal clocks). I pointed that out and that brought about more attacks… The low jitter crock (I meant to say clock) goes on, and people are clocking with external clocks a lot of stuff they do not need to.
When your AD is using the internal clock for conversion, you are doing the best you can. The data sent forward to a computer, DAW or what not, is “after the conversion” so it does not need to be clocked with special care for jitter, and a “standard” link (say AES or SPDIF) is just fine for data transfer.
There are times when you need to use external clock, and when you need to, use external, when it is a needed trade off. For example, say you want to clock 2 or more AD chassis together... But other then that, internal is the better way to go!
Regards
Dan Lavry"
ns
