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Topic: Why do we hear "clicks" and "pops" when a CD is sk (Read 4977 times) previous topic - next topic
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Why do we hear "clicks" and "pops" when a CD is sk

So topic got truncated: Why do we hear "clicks" and "pops" when a CD is skipping.

I havent really understood why there are sounds coming out when a CD is damaged, and played back on a CD player. Whats the reason for the sounds ?

I am assuming that this has a highly technical explanation to it, but Id be curious to know why it should make and audible "response sound" to it not being able to read either certain sectors og part of the CD.

Why do we hear "clicks" and "pops" when a CD is sk

Reply #1 –
The laser can't "see" (read) the data on the disc, and the cd player has to interpolate the audio of the last read sector and the next read sector. In the case of huge gaping holes of missing data, such as a nasty scratch, this can be very audible in the form of clicks and pops.

Why do we hear "clicks" and "pops" when a CD is sk

Reply #2 –
Even one 'wrong' sample can have a large effect on the frequency content:

Why do we hear "clicks" and "pops" when a CD is sk

Reply #3 –
We had a discussion along these lines a few months ago, but I can't remember which thread it happened in.

The argument that one wrong sample (by which I'm presuming you mean an inverted bit) can have a large effect doesn't stand true for an audio CD played back in a fully CD-DA compliant player. The Reed-Solomon error correction makes it a virtual impossibility for an error of this nature to ever make it as far as the output.

Cheers, Slipstreem. 

Why do we hear "clicks" and "pops" when a CD is sk

Reply #4 –
Quote
The laser can't "see" (read) the data on the disc, and the cd player has to interpolate the audio of the last read sector and the next read sector. In the case of huge gaping holes of missing data, such as a nasty scratch, this can be very audible in the form of clicks and pops.


I dont get this. So a scratch in the CD, leaving the CD player to interpolate (generate the sound between the functioning sector and the upcoming unreadable one) will leave it to make a clicking sound ?

Why do we hear "clicks" and "pops" when a CD is sk

Reply #5 –
Simple answer- because samples are missing 
Most of the errors I have seen are that the player/ripper fills the missing samples with the same value as the last good sample, so there is a "flat" line. As the next first readable sample comes, it's different value and that's where the "click" happens.

Why do we hear "clicks" and "pops" when a CD is sk

Reply #6 –
Quote from: sizetwo on
Quote
The laser can't "see" (read) the data on the disc, and the cd player has to interpolate the audio of the last read sector and the next read sector. In the case of huge gaping holes of missing data, such as a nasty scratch, this can be very audible in the form of clicks and pops.


I dont get this. So a scratch in the CD, leaving the CD player to interpolate (generate the sound between the functioning sector and the upcoming unreadable one) will leave it to make a clicking sound ?

There are several "layers" of protection agains errors.

I believe that the error-correcting codes will be able to fully correct (replace erroneous physical read values with data that is _identical_ to those of a undamaged CD) up to a maximum length through interlaving of channel symbols and Reed-Solomon codes.

If the number of errors within a time-window is too large for correction, erroneous samples may still be detected and replaced by interpolated values. The algorithm for interpolation is manufacturer-dependant, but I have seen the specs of one Philips chip that used linear interpolation if a single sample was wrong, and simply held the last value if two or more samples were wrong.

There is still a faint possibility that a sample will have an error that is not detected by the CRC. I have no idea what the probability is, but likely very low.

Apart from the signal processing, there is also the possibility that the mechanics/laser cannot reliably track the damaged CD. I guess that the behaviour is manufacturer-specific in this case as well. I suspect that this is the reason why old CDs sometimes will stutter and re-play the same short segment infinitely?


"Clicks" are typically caused by discontinuities in the stream of samples, something that is uncommon in music content.

-k

Why do we hear "clicks" and "pops" when a CD is sk

Reply #7 –
Are we talking about scratches, or are we talking about skipping?  They're not the same thing.

The pops and noise from scratches are data being interpolated, and the CD built in interpolation alg is a linear reconstructions so... fast but pretty bad. 

Skipping is the player literally losing its place, the exact analog of the needle skipping across a groove on a vinyl record.  All the way down to the fact that a CD is physically a spiral (just that the data in it is digital).  Like on an analog record, a scratch can cause the laser to skip off its groove (not as likely, but still possible).

Also, because reading is a synchronous process [albeit with a large buffer to allow wiggle room], if you don't read the bits when the drive's buffer gets full, it has to stop reading.  And then it loses its place.  Audio CDs aren't like data CDs; there's no sector sync pattern, no sector framing, and the subchannel information is spread out across little frames.  resyncing after an interruption is error-prone.  Audio CDs weren't really designed with this in mind.

And then, there are bugs.  Have you ever written a large program 100% free of bugs?  Yeah, neither have the CDROM manufacturers ;-)
 

Why do we hear "clicks" and "pops" when a CD is sk

Reply #8 –
Okey I seem to get a better understanding of it. Thanks for the replies guys.