czp@houxa.UUCP (C.PODARAS) (05/15/84)
a number of people have replied to an earlier article which i posted to the net regarding cd recommendations. in that article, i made the following statement: > if you listen to any of these on a good CD player -- one that creates > *simultaneous* (ie, no inter-channel half-sampling-period delay,like > the sony cdp 101) channel outputs thru appropriate data buffering > and use of dual D/A's, and has some semblance of flat-group-delay- > characteristic reconstruction filters -- you will (assuming you like the > music) be *amazed* at what this medium can sound like. to answer some of the questions, here goes... the standard for cd recording (ie, sony, philips, aes standards committee) calls for ** simultaneous ** sampling of audio channels; in the case of mastering for cd, this means that left and right are sampled at the same time. the l and r channel bits are then combined (interleaved) into a single stream for recording into the cd medium. for playback, this serial stream enters a buffer, allowing the left and right channel bits to be pulled apart, and hence the channels can be played back simultaneously. following the standard, then, will cause l and r output signals to be coincident in time. the problem is that some players don't have the buffers. (philips and marantz players, to mention two, are ok.) some players (ie, sony cdp101) use a single d/a converter which switches very quickly from l to r channel, causing an 11.34 us time lag between channels (half of the 22.68 us sampling period, as the converter switches back and forth to cover both channels for each period). also, some other players use separate d/a's for each channel, but still no buffering: they switch the error-corrected output very quickly between the converters' inputs during a sampling period, which still causes the half-sampling-period lag. this lag causes a slight high-frequency phase shift, which (pick your side of the fence) may be an audible degradation in stereo playback. however, there's no question that problems do exist in mono playback, where (as many broadcasters have unfortunately found out) the high-frequency cancellation is ** very ** noticeable. the solution thus far has been either to use analog delay lines, or to switch to a "good" player. check this out on your cd player by listening to a *stereo source* (ie, music disc: some test discs only have material on one track at a time) with the preamp playback mode in mono. headphones recommended. result?...bummer. don't mean to disillusion you early player owners; just remember there's always a learning curve in engineering design. ah, the old engineering "gotcha": get tricky to save $ by scrimping on parts, and marketing will love ya. then murphy comes to town and lays down his law... (i'll buy one eventually, when you don't have to tear it apart in lab to make it work) chuck podaras bell communications research holmdel, gnu joizee {..., ihnp4}!houxa!czp
ron@brl-vgr.ARPA (Ron Natalie <ron>) (05/16/84)
This is similar to the edict that one of the FM radio stations that I was associated with had. NO QUAD records. While Stereo playing of these records sounded OK, if you watched the L+R meter the whole thing was nil in mono. -Ron
michaelk@azure.UUCP (Michael Kersenbrock) (05/16/84)
<***>
In regards to DAC flopping causing phase delays, I think it needs
to be pointed out that the delay is a **constant time delay** regardless
of frequency. This means ZERO group-delay distortion or "linear-phase",
so there is NO distortion produced by this effect. But wait, there is more.
"Everyone" knows that moving your speakers around in your room
definitely makes a difference in what things sound like due to interchannel
phase differences, reflection differences, and whatever you please.
I don't know *which* channel of my CD player is the "late" channel, but
for this example I will say it is the right channel. The right channel
comes out 11uS after the time is should have. Sound travels 0.15 inch
in 11uS. If I move my right speakers forward (toward me) by that distance,
it makes all sound from that speaker system come at me 11uS earlier that
it did before I moved them. I have exactly compensated for the DAC
sampling interleave.
What does this mean? If you CAN tell the difference of moving your
speaker 0.15 inch, then you can also tell the difference between the
Sony CDP-101 (one DAC) and the Sony CDP-701 (two DACs), BUT you
can compensate by moving the slow speaker channel by 0.15 inch toward
you.
Mike Kersenbrock
Tektronix Microcomputer Development Products
Aloha, Oregongregr@tekig1.UUCP (Greg Rogers) (05/17/84)
While it is true that many CD players have an 11.34 uS time delay between channels for the reason you cited this has ABSOLUTELY NO EFFECT during normal stereo reproduction! During stereo reproduction through speakers the 11.34 uS time delay is exactly the same effect as moving the left or right speaker forward by 0.15 inch. (Sound travels approx. 1130 ft/sec. Hence 1130 ft/sec x 11.34 uS x 12 inches/ft = 0.15 inches.) Since I am quite certain you haven't measured the distance from your ear to the left and right speaker to this accuracy, and installed a head restraint to prevent you from moving your ears less than 0.15 inches while you listen, you are already listening to far more interchannel time delay than this with your ANALOG record playing system. So if this problem is really bothering you so much while listening to your speakers you don't have to buy a "good" CD player to fix it, simply move one of your D#!% speakers. Please note: This is an entirely different issue than nonlinear phase response in a single channel caused by anti-aliasing filters which we have spent so much time on in the past. I also don't believe there is any evidence to support the ability of interear time delay perception anywhere near 11 uS. This would be the case when listening to stereo headphones. With stereo headphones the path delay between ears is pretty tightly matched. But since each ear gets a single channel exclusively there is no opportunity for phase cancellation. The only case where a problem can exist is with mono signals electronically mixed together by putting a preamp in the mono mode. Even here it is necessary to use headphones to detect the effect, since with speakers each ear will hear both speakers and a path difference larger than 0.15 inch will result unless the abnormal (and perhaps kinky?) precautions mentioned above are taken. Hence you will hear both the effect from the CD player and the effect from using two speakers to reproduce mono present with any ANALOG media. For my own listening I could care less what happens in mono through headphones and wouldn't spend a dime more to get a player that corrects a problem in a mode that 99% of the user's would never use. To me this is GOOD engineering not bad as implied. Radio station's that broadcast in mono may be well advised to buy a unit prepared for this purpose however. Once more into the breach, Greg Rogers Tektronix