By Julian Hirsch
Reading the Mail
READER mail is a valuable source of feedback, letting me know what your problems are and whether I am answering your unspoken questions-or perhaps shedding more confusion than light on some of the murkier aspects of hi-fi. Although I read every letter, I am unable to answer most of them. (Remember, I cannot give specific product or installation advice or answer any letter that isn't accompanied by a stamped, self-addressed envelope.) Some correspondents, fortunately, bring up points that merit a re ply, either because something I have written is unclear or because the writer's experience augments or contradicts my own. For example, one reader (among many, I'm sure) wonders about the concept of amplifier "headroom." His problem arose because a dealer advised him against buying a certain receiver because, he said, it had only a 0.5-dB headroom (he did not say what type of headroom rating this was). The dealer claimed that at least a 3-dB headroom is necessary to do justice to digital recordings (which is not necessarily true). Having found few units advertised as having a 3-dB headroom, my correspondent was understandably confused.
In the first place, there are two types of headroom defined by the EIA's amplifier test standard, and they pertain to rather different qualities of an amplifier. Clipping head room is a measure of the manufacturer's conservatism in rating the amplifier; it compares the rated continuous power with the actual continuous power. If an amp is rated at 200 watts and actually de livers precisely 200 watts at the clip ping point, it has a 0-dB clipping headroom. On the other hand, if the manufacturer gives the same amplifier a 100-watt rating, it can then be said to have a 3-dB clipping headroom (the actual 200 watts is 3 dB higher than the rated 100 watts).
Obviously, if a manufacturer wishes to remain competitive, he will not rate his products too conservatively, despite the "advantage" of higher clipping-headroom readings.
Dynamic headroom, on the other hand, tells us something about the amplifier's ability to deliver current to the loudspeakers and, secondarily, about the regulation of the amplifier's power supply. If an amplifier is called on to deliver high peak power levels for short periods, it is quite possible, and desirable, for it to exceed its rated continuous (clipping) power output. If the amp's power supply is "loosely" regulated, there will be a relatively high volt age (more than is required for the amplifier to deliver its rated power) available under most normal listening conditions, when the average power output is likely to be a few watts or less. Even with loose regulation, this voltage does not change very much during transient high-current demands, and therefore the amplifier can deliver more than its rated output power during musical transients. Usually this short-term peak (dynamic) output is on the order of 1 or 2 dB greater than the rated output power, but in some amplifiers it can reach 3 or even 4 dB. The difference is the dynamic headroom.
It would be misleading to attribute too much significance to this quality. After all, if the amplifier clips audibly, you need merely turn the volume down slightly. Nevertheless, most of us would prefer to be able to play music a bit louder than usual on occasion without fear of clipping or possible damage to our speakers. The matter can be summed up in this way: of two amplifiers having the same clipping-power output, the one with the greater dynamic headroom will be able to sustain a louder average level without distortion. On the other hand, if two amplifiers have the same dynamic headroom, the one with the higher clipping-power rating would be the "more powerful" amplifier-and probably larger, heavier, and more expensive as well. Unfortunately, dynamic-head room specs are not always given in manufacturers' literature, so you may have to depend on test-report results for that specification.
On a somewhat related matter, another reader expressed serious doubts about the ability of an amplifier with a "smart," signal-con trolled power supply (such as some of those from Carver, Soundcraftsmen, Hitachi, Yamaha, and others) to respond fast enough to avoid clipping the leading edge of a transient signal. Without corroborating evidence, he states that such clipping distortion is unavoidable and audibly serious, and he even suggests a method of testing for it.
As it happens, the standard EIA dynamic-headroom measurement is done in much the same way as the test this reader proposes, with 20-millisecond bursts of a 1,000-Hz sine-wave signal. I have tested quite a number of "smart" amplifiers in just this way, and I have never found any sign of such a problem.
The reason, of course, is that the power-supply response time has been made shorter than that of the amplifier, so that it is never "caught short." Like so many other audio "problems" we hear about, this one does not exist in the real world.
Another overrated "problem" although certainly a highly controversial one at the moment-concerns the effects of using a single digital-to-analog converter (DAC or D/A) to generate the two output signals of some CD players. In such products, the converter output is switched (multiplexed) between the left- and right-channel signals, with the result that the analog signals do not emerge exactly in phase with each other (assuming that both channels were in phase on the recording). In contrast, a player that has separate DAC's for each channel can deliver outputs that are almost exactly in phase at all audible frequencies. The reader who wrote me about this asked why I do not comment on this property of the CD players we test and on its audible significance, if any. Note that the phase shift we are discussing is between the two channels, or inter-channel, and can be thought of as a constant time delay on one channel.
It is not the same as the intra-channel phase shifts caused by CD-player output filters (see theis article).
----If an amplifier is called on to deliver high peak power levels for short periods, it is quite possible, and desirable, for it to exceed its rated continuous (clipping) power output.---
As a matter of fact, I do comment on it. Whenever we test a CD player, the phase shift between channels at 20,000 Hz is mentioned in our report. If the phase shift is small, on the order of 5 to 10 degrees, it is a fair assumption that the player has separate D/A converters (or one DAC and a corrective time delay on one channel, which amounts to the same thing).
A single multiplexed and uncorrected converter will produce a phase shift of about 90 degrees at 20,000 Hz. Almost all CD players we have tested fall into one of these two categories.
Such a "horrendous" phase shift as 90 degrees, occurring at a frequency well above the normal range of human hearing, corresponds to a time difference between the channels of some 11.3 microseconds. If you are listening in mono, or to a stereo broadcast in which the two channels are encoded as sum (L + R) and difference (L-R) signals, a 90-degree phase shift can have a very slight effect on frequency response. But to put it into an acoustic perspective, the effect is equivalent to a difference of about I/8 inch in the distance from each of the speakers to the listener's ears.
If you think you hear any effects from a CD player's phase shift in normal stereo listening, try moving one ear about 1/8 inch closer to one speaker. If the sound gets Worse, approach the other speaker with the other ear (be sure to move 1/4 inch, to compensate for the first movement!). I have yet to hear any audibly detectable difference between CD players having far greater measurable differences than this, and I would suggest that there are more important problems to worry about in the never-ending search for perfect sound reproduction.
And, finally, I will try once again to explain why I sometimes use language that seems to some readers to avoid coming to grips with the issues as they see them. Actually, I generally take great care to say just what I want to say, no more and no less. Perhaps I am not always successful, but that is my goal. One gentleman takes me to task for our February report on the Pioneer CLD-900 combination video-disc/ Compact Disc player. He was disturbed because I said that "a Laser-Disc ... even in its CX-encoded analog form, can have ... quality superior to ... conventional home video-cassette recorders" (emphasis added). For reasons that are not clear to me, he seemed to take exception to the two words italicized in the quotation. I said "can" because I have not, obviously, personally evaluated every LaserDisc player and VCR, which would certainly be required before I could say "does" instead of "can." I said "conventional" to exclude hi-fi VCR's, which can (that word again!) have better audio quality than a CX-encoded LaserDisc.
My correspondent, however, dismisses my carefully worded statement as a "mouthful of nothing," concluding that my words were carefully chosen only to avoid of fending an advertiser rather than in the interests of accuracy. I suspect he is one of those people who prefer criticism to be sweeping and absolute even when its basis is sketchy or even nonexistent. My criticism of some of the human-engineering aspects of this product was apparently not vitriolic enough to satisfy him.
Sorry about that! If you must have capricious product criticism, be warned that I'm making every effort to disappoint you.
Also see: THE HIGH END (Jul. 1985)
Technical Talk By Julian D. Hirsch--Upgrading Your Stereo System Part 3 (May, 1983)