Techmoan video: Shaving Compact Discs to improve the sound

[pcprogrammer]

Let me rephrase it then. Are there markers or some metadata on an audio CD, based on which the computer can tell it is reading audio data and treat it differently then digital data.

The point was about not needing audacity to do the comparison between the two CD's, and just read the data from the CD with the computer. And there I wondered if the computer could do the same error correction as a CD player uses.

[Fixpoint]

Are there markers or some metadata on an audio CD, based on which the computer can tell it is reading audio data and treat it differently then digital data.

Yes. Audio CDs adhere to the Red Book standard.

And there I wondered if the computer could do the same error correction as a CD player uses.

Yes, it can & does. If you insert an audio CD into your drive, the hardware will detect that it is an audio CD. The driver software will then suggest to you to play it as an audio CD. However, you can write software that tells the driver "no, please read the CD as non-audio data" which will put the hardware in a different readout mode (fast than 1x speed, different kind of error correction etc).

[wraper]

Let me rephrase it then. Are there markers or some metadata on an audio CD, based on which the computer can tell it is reading audio data and treat it differently then digital data.

The point was about not needing audacity to do the comparison between the two CD's, and just read the data from the CD with the computer. And there I wondered if the computer could do the same error correction as a CD player uses.

Audio CD and data CD are entirely different formats. Audio CD has no file system. The problem with audio CD format is that a compromise of data integrity was made which allows a bit more usable data to be stored (data CD can store less data), but error correction used is quite weak and does not provide guarantee there was no error during reading. Good audio ripping programs like Exact Audio Copy read audio CD multiple times at slow speed to ensure good data integrity.
Actually compromise is understandable. Early CD audio players had no data buffer (it would cost insane money in early 80's) and read at 1x speed, therefore even if reading error could be detected for sure, player would not be able to try reading it again without interrupting the sound.

[Ed.Kloonk]

Gee, I'm dusting off my memory banks here. From what I remember...

The ATAPI interface got sent back different sized packets depending on whether or not it was an audio CD.

The first gen cdrom readers were terrible at aligning the blocks to the exact position requested by the driver. The error correction for byte misalignment with filesystem blocks was done by the driver, I believe. Later the raw CDDA blocks had to be manually aligned with specific software. Better drives (2nd gen) such as Plextor which I owned a few of, fixed this CDDA read-back alignment issue.

Yet, the error correction of actual corrupted data read off the CD surface whatever the format was accomplished by the drive's own firmware. Early ones were hit and miss in how well they dealt with read-back problems. You got what you paid for.

[golden_labels]

error correction used is quite weak and does not provide guarantee there was no error during reading

As a minor correction: data CDs have no such guarantee either. The difference is quantitative, not qualitative.

Mode 1 sectors⁽¹⁾ of “data” CDs (CD-ROM, yellow book) offer higher resilience against data corruption than “audio” CDs (CD-DA, red book). But they are not immune to read errors in yes-no sense; not even in “almost surely” sense. Merely “less likely”, which is also diminished by much higher read speeds.

The amount of data, that contributes to read errors being detected on data CDs, is also minuscule: less than 0.2% of the capacity. The rest is used for data encoding, but only that 0.2% is used to detect errors.

While standard audio CDs are indeed having a bit more capacity than data CDs, the massive advantage you may see in some releases is achieved in a different manner. Since audio CDs are read at much lower speeds and read errors are not a critical issue, they can push track separation to its limits, beyond what is permitted by the standard, and pack more tracks into unit space.

⁽¹⁾ But not Mode 2 sectors. Those are still taking slightly more space, but without added protection.

[jonovid]

seen the video-
in my opinion the platter drive machine can be modified for disc polishing out heavy scratchs on the play surface , here is were the
noticeable difference between data and no data is realized.  not rim trimming!
as most disc repair machines are not as robust as needed to hold the disc when applying pressure when polishing.

[Jeff eelcr]

CD's are not all manufactured perfectly I have seen quite a few that had to have the
edges shaved in order to work correctly (extra plastic on edges) can cause rubbing
and jaming problems in changers if not the correct size.
Jeff

[Nominal Animal]

what is the quantitative relationship between the required bit depth and the overall dynamic range?

They're basically the same thing, about 130 dB ≃ 21 bits.

Human hearing is based on small tuned antennae (stereocilia connected to cochlear hair cells) which have roughly logarithmic response.  Each cell is activated by a specific frequency, and there are around 15,000 of them in each ear.  The amplitude range at which these respond to sound varies based on the frequency, but in the middle frequencies (with most cells), the dynamic range is 120-130 decibels, corresponding to 20-21 bits.

The detail where required bit depth and dynamic range differ, is when we combine a loud sound with a quiet sound: especially low frequency loud sounds override quieter higher frequency sounds.  This is why dynamic compression "works": by having the signal "clip", the effect is similar to how in the ear, loud low frequency signals "hide/silence" higher frequency signals.

[Zero999]

The dynamic range of the human hearing might be 120 to 130dB, but it's not safe to be exposed to sound at the higher end of this range. The dynamic range of a CD at 96dB is more than sufficient to cover the dynamic range of sound which is safe to be exposed to.

[Nominal Animal]

Human perception of sound is wildly strange, though.  Now that you know that the human hearing organ is basically a spectrogram with 13,000 or so bins and 100+ dB of dynamic range, think about how it is possible for a person (with hearing only in a single ear) to be able to listen to two different people at a time?  Or even detect the voice of someone familiar in a babbling crowd?  I'm tempted to say the human brain is a pattern detector, but its is so much more than that, because in detecting a voice (be it human, animal, event, musical instrument, or whatever), the time component is also involved; and it can detect a huge amount of different sounds at a time, even in parallel –– like running water, birdsong, leaves rustling, and two people talking at the same time.

It is even possible to generate sound that is perceived as quieter than silence.  This sound is just shaped noise modeling the sensitivity of sound perception, i.e. the human psychoacoustic model.  To the brain, it contains nothing interesting, and just shifts the "base" threshold of detection, so that even sounds generated by our own bodies (heartbeat, blood flow in veins) get attenuated: everything becomes "more quiet".

In audio compression, the psychoacoustic model has been used ever since MP3 came to be, to distribute the quantization noise, yielding better perception of the sound.  (Essentially, it is about controlling what information is conveyed, and where in the spectrum the errors/noise due to compression and quantization is placed in.)

Because of the brain, I bet that things like lighting (as in whether you are in a dimly-lit room with soft yellow-orange lights, or harsh cold blue-white light) and especially scents have more effect on the experience than things like exactly which amplifier or amplifier settings you use.

[AVGresponding]

 





 

[PlainName]

They still have a metallic layer, don't they? Ergo magnets must affect them, and anything that does that affects the nois sound.

I think they've missed a trick, though. They should supply 'magnets' with a sticky surface so you can see they're effective when they lift a CD.

[Nominal Animal]

If I didn't have scruples, I'd start selling audio-enhancing candles for the audiophɪ̥̊les.  The sales pitch would extol the enhanced air carrier mechanisms for the audio waves, producing a richer listening experience.

[PlainName]

Ding!

Apparently A UK museum are experimenting with smells to enhance the experience. So you could sell 'smellopacks' for DVD films, so that when some particular scene is playing, the viewer can smell what's on the screen.

Of course, it's only once they've bought the pack that they find out it's a box of candles and they have to manually light and extinguish the appropriate one at the right times. But they'll get a timesheet to help them.

[golden_labels]

You are late with that idea: 4D cinema. Horrible experience!

[PlainName]

But they cost a fortune set up. A box of scented tealight candles are far cheaper (but should still sell at a premium price).

[AndyBeez]

Degaussing CDs... 100% Dodgy Technology. If CDs were Magneto Optical Drives (MODs), then maybe this might boost 'rewrite integrity'.  One test could be holding an AM radio near to the box and listening to the EM field. Or use one of @Eevblog Dave's home made EM probes. If it actually produces any EM field 

+ or just place a compass needle on the box and see if it spins. Bets are, nothing happens.

@Techmoan, I would love to see the shkematic for the PCB. Looks like someone put a lot of thought into this box with it's fading green LED

[coppice]

You are late with that idea: 4D cinema. Horrible experience!

The ones I've experienced in Disneyland were good fun. Not something you'd want regularly, but a fun occasional experience.

[Ben321]

I wonder about that interpolation bit if that also applies for a CD reader in your computer when it reads "digital data" and not "audio data". I assume that it will only do this interpolation when it has read part of the audio data stream, detects that there are errors, tries to resolve the errors, can't and will then interpolate between previous and next samples.

Yes, that's how I understand it.  I should have said "audio CDs".  I honestly don't know what type of error correction data CDs use, but there is a spec for it (Yellow Book?)  I can't imagine that they would do interpolation on data.

Data CDs use an extra layer of FEC (in addition to the standard RS FEC) that uses a technique other than RS (I think it's convolutional coding or something). This is done per sector of data. Plus I think it also stores a checksum or CRC after each sector so it can tell if the data is correct after all FEC corrections have been performed when reading the disk. In fact, because of these additional FEC bytes and check bytes, there's actually less space available on a data CD than an audio CD.

[Ed.Kloonk]

From the wiki:
https://en.wikipedia.org/wiki/CD-ROM

Both Mode 1 and 2 sectors use the first 16 bytes for header information, but differ in the remaining 2,336 bytes due to the use of error correction bytes. Unlike an audio CD, a CD-ROM cannot rely on error concealment by interpolation; a higher reliability of the retrieved data is required. To achieve improved error correction and detection, Mode 1, used mostly for digital data, adds a 32-bit cyclic redundancy check (CRC) code for error detection, and a third layer of Reed–Solomon error correction[n 1] using a Reed-Solomon Product-like Code (RSPC). Mode 1 therefore contains 288 bytes per sector for error detection and correction, leaving 2,048 bytes per sector available for data. Mode 2, which is more appropriate for image or video data (where perfect reliability may be a little bit less important), contains no additional error detection or correction bytes, having therefore 2,336 available data bytes per sector. Note that both modes, like audio CDs, still benefit from the lower layers of error correction at the frame level

When a disc image of a CD-ROM is created, this can be done in either "raw" mode (extracting 2,352 bytes per sector, independent of the internal structure), or obtaining only the sector's useful data (2,048/2,336/2,352/2,324 bytes depending on the CD-ROM mode). The file size of a disc image created in raw mode is always a multiple of 2,352 bytes (the size of a block).[20] Disc image formats that store raw CD-ROM sectors include CCD/IMG, CUE/BIN, and MDS/MDF. The size of a disc image created from the data in the sectors will depend on the type of sectors it is using. For example, if a CD-ROM mode 1 image is created by extracting only each sector's data, its size will be a multiple of 2,048; this is usually the case for ISO disc images.

On a 74-minute CD-R, it is possible to fit larger disc images using raw mode, up to 333,000 × 2,352 = 783,216,000 bytes (~747 MB). This is the upper limit for raw images created on a 74 min or ≈650 MB Red Book CD. The 14.8% increase is due to the discarding of error correction data.

Code: [Select]

Format ← 2,352 byte sector structure →
CD digital audio: 2,352 (Digital audio)
CD-ROM Mode 1: 12 (Sync pattern) 3 (Address) 1 (Mode, 0x01) 2,048 (Data) 4 (Error detection) 8 (Reserved, zero) 276 (Error correction)
CD-ROM Mode 2: 12 (Sync pattern) 3 (Address) 1 (Mode, 0x02) 2,336 (Data)