AES Journal Forum Comments

AES Journal Forum: Comment by Douglas Rife on "Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback"

Mon, 27 Oct 2008 12:12:33 -0600

Title: Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback
JAES Volume 55 Issue 9 pp. 775-779; September 2007
Comment by: Douglas Rife

Mr. Meyer: Thank you very much for the additional information regarding the CDR-850. Another clue you did not mention would be to measure the input-output impulse response (IR). A large delay of several milliseconds or more would be expected for a digital signal path due to decimation and reconstruction FIR filters. Furthermore, such FIR filters, as you know, are typically linear phase, exhibiting symmetrical (in time) ringing behavior in their IR. The noise floor measurement needs to be made with full scale sinewave input to elicit the quantization noise of all the available bits of resolution. A statement from the manufacturer is also needed as confirmation. I'm surprised that my rather obvious question was not asked and answered long ago, indeed, well before the publication of your report. Doug Rife

AES Journal Forum: Comment by E. Brad Meyer on "Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback"

Thu, 23 Oct 2008 11:47:45 -0600

Title: Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback
JAES Volume 55 Issue 9 pp. 775-779; September 2007
Comment by: E. Brad Meyer

Mr. Rife; Of course, the CDR-850 does what we said. If you had it in your system you would quickly realize this.

If you just send a signal to the analog inputs and advance the record volume control, there is no indication on the level meters and no audio comes out of the output.

With no disc inside, pressing the RECORD button puts the machine in MONITOR mode (as the user manual says, without spelling out specifically what this means). The meters move, and signal appears at the output. That the audio is coming from the D/A converter is shown by several things, most notably (1) the visible and audible hard clipping that occurs simultaneously with the activation of the overload lights, (2) the noise floor at 92 dBA below the clipping level, (3) a slight but (with very good equipment) measurable frequency response error, on the order of +0.2 dB between about 9 and 11 kHz (if memory serves), and (4) the very steep roll-off above 20 kHz due to the anti-aliasing filter. Good modern preamplifier stages are at least 20 dB quieter than this, and you would have to try hard to design one that performs as badly in just these ways.

Meanwhile, under the assumption that the previous is inadequate to prove the case to you, I am obtaining the block and circuit diagrams from the service manual. I talked to the service tech at Sennheiser in Connecticut, who says that yes, this is the way it works, but I have an email into the engineering staff in England for a definitive quote to post here, for the record. — E. Brad Meyer

AES Journal Forum: Comment by Douglas Rife on "Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback"

Mon, 20 Oct 2008 18:31:14 -0600

Title: Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback
JAES Volume 55 Issue 9 pp. 775-779; September 2007
Comment by: Douglas Rife

Of course, I'm serious about this. Let's be clear that IF a recording is burned to a CD and subsequently that CD is played back from the start, there would be little doubt the resolution was being reduced to 16/44. However, that method would be quite cumbersome in practice as it would require pre-recording the hi res source to CD, "rewinding" both source and CD and then restarting the playback of the source in exact synchronization with the CD playback. Thus, as your paper strongly implies, you used the CD recorder's real time monitor function, which avoids the synchronization problem altogether since, being real time, the audio signal would not be delayed by more than a few tens of milliseconds and possibly much less. However, the real time monitoring function could very well be performed completely in the analog domain, without ever being converted to the digital domain! If that is the case then your A/B listening tests prove only that the analog signal path of this CD recorder is very transparent. Note that in the case of analog tape recorders having separate record and playback heads, it's easy to monitor the actual recording taken right off the tape in near real time. But CD drives do not normally allow for this type of monitoring so a CD recorder's monitor function could be implemented purely in the analog domain or, possibly in the digital domain but at a higher resolution than 16/44. Most A/D converters operate at a much higher resolution than 16/44. The reduction to 16/44 need not be done until the data is actually burned onto the CD. There is no requirement I've ever heard of that the real time monitor functions of CD recorders reduce the resolution to 16/44. That is what needs to be demonstrated but you can't use personal listening to do that since you are trying to test the audibility of lowering the resolution to 16/44 using controlled listening tests with many listeners and with several different hi res audio playback systems.

AES Journal Forum: Comment by David R. Moran on "Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback"

Sun, 19 Oct 2008 12:15:41 -0600

Title: Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback
JAES Volume 55 Issue 9 pp. 775-779; September 2007
Comment by: David R. Moran

>> that this CD recorder's monitor function reduces the resolution to 16 bits and the sampling rate to 44.1 kHz ... needs to be proven beyond all doubt if your [sic] going to publish a paper on such a controversial subject. ... Don't forget Murphy's law.

Assuming you are being serious here and that this is a serious worry being expressed, do you think it possible that the CDR we used has *better* performance than 16/44? In other words, and on the other hand, if it is of crummy performance, say, only 14-15 bits and 40k bandwidth or some such, what would that show? It would show that hi-rez can be degraded *way below* the CD standard and still be audibly indistinguishable from hi-rez. Uh, is that what you intended? I mean, think about it. Which direction does Murphy work in here?

I will ask Brad Meyer to weigh in about the performance of this recorder, what he has measured, if he has, and what the official spex are. I wonder if this will be satisfactory. Murphy did not make a law about paranoia!

The only review I could quickly find with a search reported:

\\\ I checked the performance of the built-in sample-rate converter with a varispeeded CD source and found it tracked extremely well, as it also did with both 32 and 48kHz stable signals. I also made several test recordings of familiar material copied from commercial CDs, both with and without the SRC in circuit (on adjacent tracks in some cases), but could not hear any significant difference. To my ears, and those of my friends and colleagues, all of the copies made on the CDR850 were indistinguishable from the source material when played back on the source machine (material was copied mainly from a Meridian 508 CD player using the optical or co-ax connections, co-ax giving the best results).
(Hugh Robjohns, http://www.soundonsound.com/sos/may99/articles/hhb.htm, 5/99)

I wonder if this is persuasive enough testimony. In any case, surely you do not think the unit might possibly exceed the CD standard, do you?

AES Journal Forum: Comment by Douglas Rife on "Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback"

Sat, 18 Oct 2008 17:06:08 -0600

Title: Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback
JAES Volume 55 Issue 9 pp. 775-779; September 2007
Comment by: Douglas Rife

Mr. Moran:
Thanks for the link above to the Boston Audio Society.
The HHB CDR-850 professional CD recorder was used as the A/D/A block. Fine.
Still, I found nothing in that link demonstating that this CD recorder's monitor function reduces the resolution to 16 bits and the sampling rate to 44.1 kHz. It's a reasonable assumption but, as I said in my original comment, it needs to be proven beyond all doubt if your going to publish a paper on such a controversial subject. The HHB web site does not even include the basic user's manual for this model. Don't forget Murphy's law. Don't forget the Hubble space telescope's main mirror, that everyone knew was the correct shape.