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This is just silly

Posted by Charles Hansen (M) on December 22, 2005 at 09:08:41

In Reply to: Perhaps it is you who has reading comprehension issues? posted by Christine Tham on December 22, 2005 at 00:59:31:

You are correct that the line I quoted can be interpreted in two different ways, thereby reaching two different conclusions. However, I am confident that my interpretation is the correct one, based on the following facts:

a) There was absolutely no reason for them to be ambiguous with these claims. In fact it would have been a great marketing advantage for them to say "The PMD-100 has three different digital filters -- two that dynamically switch when playing back HDCD-encoded discs, and a separate one for playing standard CDs." But instead they chose to put a very vague and ambiguous reference in *one* single document, and leave out all mention of this feature in all other literature, data sheets, marketing materials, technical presentations, et cetera. Given Pacific Microsonics' known track record of misleading marketing, this is pretty damning evidence.

b) At the time the PMD-100 was introduced, there was a practical limit as to how much circuitry could be stuffed into a single chip. A digital filter consumes a lot of hardware resources, and the one digital filter known to be inside the PMD-100(that is detailed in the datasheet) was by far the biggest one used at that time. (It's specs were far superior to other similar parts of that time, with greater stop-band rejection and a larger number of taps.) It is extremely unlikely that Pacific Microsonics could have fit *three* digital filters, plus the sub-code detection circuitry and associated timers, plus the expansion circuitry, plus all of the other housekeeping circuitry required to run the lights and gain scaling, et cetera.

Now I realize that neither of the above facts represent incontrovertible proof. (Which is why we are still having this argument.) One needs to examine the facts and draw conclusions. While it is possible that I have made an incorrect conclusion, I don't think this is the case. However in the absence of some additional evidence surfacing, I don't think we are likely to make much headway in this scenario. Perhaps it is best to agree to disagree.

Regarding the signal level issue, you are simply wrong. If you understood the compansion process used in HDCD a little better, you would realize your error. I will try to explain this to you.

If "peak extend" is turned on during the mastering process, then the overall signal level is increased by 6 dB. To avoid digital "clipping", the top 9 dB of the incoming audio signal is squeezed into the top 3 dB of space on the disc. During playback, the overall signal level is reduced by 6 dB (hence the requirement for gain scaling of discs encoded using "peak extend") and the upper 3 dB on the disc is unsqueezed back to the original upper 9 dB of audio signal.

Please note that the above compansion process is (almost) completely transparent. (There is a negligible increase in the quantization error of high level signals.) If you have found a disc with the restored audio signal having nothing above -6 dBFS, all that means is there was nothing above -6 dBFS in the original audio signal. The exact same would be true if the disc were recorded and played without any HDCD processing. So your claim of being able to identify discs that use "peak extend" based on the peak signal level is simply incorrect.

As far as designing equipment today, your claim that "[HDCD] chips are easily available and at low cost" is also incorrect. There are *no* stand-alone HDCD decoder chips available today, and there haven't been for many years. If we discount the possibility of finding a limited stash of NOS chips in some back alley, the only choice is to use a DSP chip.

A DSP chip that decodes HDCD is not "easily available". In other words, one cannot just purchase it from Digikey or as an off-the-shelf part from a distributor as one can with a non-HDCD digital filter. Instead one has to purchase an HDCD license from Microsoft, which $10,000 plus pages of forms. (I've never applied for an HDCD license, but for example a Dolby license requires a complete financial statement of the company. Just filling out the paperwork for a Dolby license takes about a week.) Once you are licensed, then you can buy DSP chips with the HDCD decoding algorithm. Of course then you need to write the program for the other functions that the chip will perform, such as digital filtering. Then the chips themselves need to be programmed. So, yes, HDCD decoding chips are "available", but not "easily".

A DSP chip that decodes HDCD is also not a "low cost" item. These chips are designed for surround-sound processors, not CD players. They are *much* more powerful than is required for HDCD decoding. They will add significantly to the bill of materials of a CD player.

The above should be self-evident, based on the fact that so few CD players offer HDCD decoding. In the '90s, most high-end CD players *did* offer HDCD decoding precisely because the HDCD chip *was* "readily available at low cost". The only barrier to entry back then was the license fee (which was $5,000 then).

When we design a product, we make the decisions that we feel will create the best product we can make, balancing all relevant factors. The decisions we make are based upon the actual situation at that time. If we were to design a CD player today we might reach a different decision than we would have four years ago, as the circumstances have changed. But even in a cost-no-object design, we might not choose to implement HDCD decoding. For example, one problem is that DSP chips operate with extremely high internal clock speeds. For example, the current generation of SHARCs run at 400 MHz. This creates problems with internally radiated RFI inside the player, and can degrade the sonic performance of the unit. All engineering is a series of tradeoffs and compromises. Each designer will reach a different conclusion. The consumer is the one who gets to decide which design best suits their needs. If you think that another manufacturer has made the best decision by including HDCD decoding in their products, then you should purchase their products.

This post is made possible by the generous support of people like you and our sponsors:

Analog Engineering Associates

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