I'm not sure if people are aware how "checksums" work but what you do is to split a signal into chunks, say 8 bits, then you add up the "1s" (or "0s") and add an extra "1" or "0" to the signal so that each set of now 9 bits is always even (or odd). So for example 01010101 for an even checksum becomes 010101010 (four 1s) and for an odd checksum becomes 010101011 (five 1s). Checksums are as old as computing and indeed if you get a memory problem on your pc you will often get an error message "Checksum error".
However, to be 100% clear, if a checksum error is identified there is no way that anyone can know which of the bits is wrong, indeed it can be more than one bit and even the checksum bit itself. There is absolutely no way that this can be corrected except by repeatedly reading the signal and hoping the error will go away. Read errors on CD drives ahould be very infrequent anyway because, as another poster has pointed out, they are only looking for a pit or the absence of a pit.
To summarise, whilst in theory it is possible that a ripped version of a track would be better than a "played" version, in practice the difference will be marginal at best.
PS It always amuses me when people quote HiFi firms in terms of technical expertise. The link that CNo posted reads like it was written by somebody who has just read a childrens' encyclopaedia which indeed is about the level of expertise I would expect. (I was no circuits man myself but none of the good ones ever went into HiFi.)
A computer drive will go back time and time again (if needed) over the same section until it gets enough data to be 100% accurate, error correction is not required.
Until the disc is damaged and then it is. Error correction is a selectable option when ripping.
I rip without, normal discs used and abused in the normal manner, and get around a couple of percent failure.
Given the redundances in the data on the disc I find my ripping software digs out sufficient data to achieve an accurate checksum, I can recall having to use error correction on one, maybe two occasions.
If you use error correction and there is a problem with the disc, the rip will potentially stall or at least slow down. This is an indication of a potential uncorrectable error that will result in an audible artifact, such as a click on playback. Leaving error correction off, leaves in these clicks and pops and they are there to tick you off when you are least expecting it. I have (much to my annoyance) got some of these non EC'd lurkers in my collection and weed them out occasionally.
Some discs have the data on the surface under the label print and even minor scratches to this layer will destroy the data and make it unreadable. I have had several of these discs and make sure that they are replaced first and then reripped. The damage is not always easily evident and can be present without damage to the shiny/read side of the disc.
Mac mini > AVI ADM9Ts
As ever on this forum, a little knowledge.....
Data Is stored on a CD using a redundant self correcting encoding sceme known as a hamming code. The specific system is called Reed-Solomon encoding. Data is stored as blocks, and depending on the level of redundancy in the data, a given number of errors can be detected, and a given (smaller number) or errors corrected. The enthusiastic student should look up Reed - Solomon on Wikipedia.
The more redundancy in the data, the more robust the disk, but the less data can be stored. That's why yellow book CDroms (which have a higher data redundancy) store less data than redbook music CDs.
So, for a CD player doing a one pass 'on the fly ' read, the hardware error correction in the drive will attempt to correct any errors encountered. If the level of corruption is too high, it will be able to detect the error, but not fix it. In this case, the drive will either leave the value of the errored sample the same as the previous sample, or perform a linear interpolation to the next known good sample and make the missing data up (too old to remember which).
This is where a rip and a CD player differ. If the hardware error correction can't fix the error, a CD player has to move on, in contrast, if the controlling software of a rip receives an error flag from the drive, it can ask the drive to have another go. CD drives are mechanical systems, in particular the stepper drive to the laser head has backlash, and 'having another go' can place the laser in a slightly different position relative to the disc which allows a successful read to take place. This is why you sometimes hear a drive on a PC sawing away as repeated attempts to read an errored section are made by repositioning the laser read head.
So, it is quite possible for a rip to be able to extract more correct data from a cd than a CD player, doesn't make the music from the rip any better than the original recording, just less wrong than the CD player.
A note about the 'checksum'. This has nothing to do with the error correction on the drive, but is an overall test of the entire track calculated by the ripping software. It's a lot more complicated than Chris's example above - (google cyclic redundancy check) but it performs the same function. If two rips have the same checksum, then it is likely that the data is the same - this is a final test of accuracy of the rip, and if in error, some ripping program's will have another go at the entire track.
I was keeping it simple.
Thanks Andyjm, as concise a summary as I have seen.
I have to say that my maths is no longer good enough to follow all the nuances of this subject, but a basic understanding, particularly of what can and can not be achieved, can be of great help to the enthusiast in helping him understand what he is hearing and why.
One thing not mentioned in the above posts is the audibility or otherwise of error correction. I used to think that it was a possible cause for some of the differences heard between different real time transports, but now, having done a little more research, I am less sure. The suggestion is that the errors from the error correction are not correlated, ie they are not directly related to the signal, so the idea that it might cause the harshness or other identifiable traits in some players may well be misguided.
There is however one undisputed fact about digital music reproduction that applys to all 'amature' enthusiasts and music lovers, and it is this.....
"The moment you think you understand what is going on in digital audio, you are wrong".
This must be the most counter intuitive subject that I have ever come across in real life.
We do so many shows in a row,
And these towns all look the same,
We just pass the time in our hotel room
And wander 'round backstage,
Till the lights come up, and we hear that crowd,
And we remember why we came.
Well, after all the fur and feathers have settled, I am not going back to CD replay as the only digital source. My current set-up has CD replay as an option (and so might whatever I upgrade to) but I don't use it much because my CD collection is boxed and stored in a cupboard under the stairs!
I bought (and ripped) nine CDs yesterday - a box set - and will continue to buy (and rip) CDs in favour of downloads. I am not bothered if my ALAC and 320K AAC rips upset anyone here because I use technology to suit my purposes rather than to do battle in futile arguments about 'ultimate quality' that have going on since the (equally stupid and futile) CD vs vinyl arguments that started in back in 1983 (or thereabouts).
Interesting (in relation to the discussion of cheap PC drives) to note that some £5000+ CD players are using cheap, OEM, CD-ROM drives with noisy loading mechanisms...
... it runs at 4 X normal speed and uses an embedded Intel based computer (running Linux) to read/re-read the data and run error correction as necessary.
So what is the essential difference between a CD rip and a CD played by this $5000 machine? One is stored permanently as a file and played on demand, one isn't stored permanently after being played.
Marantz M-CR603 + AirPlay • Rega R3 loudspeakers • iPhone 5 • iMac • Apple Airport Extreme 802.11n • Apple iPad Mini • Panasonic TX-L32D25B • Sony BDP-S390 • Ruark Audio R1 Deluxe • Humax HDR-Fox T2
Still gets 5 stars though
:rofl: good god, and people wonder why hifi bods get called gullible. there has to be about 500-800 quid worth of parts in that maximum, so the mark up is astronomical, and the advertising bumf is unbelievable, given they've basically taken a pc and now called it "a new method for playing cd's"
Thanks for that post. I didn't know that computer CD drive mechanisms could try rereading the data from different angles when necessary. I've learnt something new today already.
PC > AVI Neutron Five 2.1
Sony NWZ-A847 64GB Walkman > Westone UM3x
Some audiophiles refuse to believe (or understand) how a £15 CD drive in a dirt cheap PC using a digital output into a competent DAC will play CDs better than an expensive audiophile CD player.
Ironically the WHF review of that Parasoiund Halo CD player explains why perfectly. Although why they need to charge £5000 for a cheap Linux PC in a fancy case I don't know.
"The Halo disc drive runs at four times normal speed, allowing it to read sections of the disc many times to minimise reading errors. Every data sector is initially read twice; those two reads are then compared. If they match, the data is sent to a large memory buffer. If they don’t, the process is repeated until the buffer (which stores around 30 seconds of music) is about to run out. Only at that point is error correction employed"
Similar to any car cd player then at 20x the cost. Seems fair.
Mordaunt Short Mezzo System C - 8,5,1,9.Yamaha V2065. SonyS570. Panasonic 42in., Sky HD 1TB. Garrard 86SB. PF30. Wii. WDTV Live. Harmony One. STAX300. QED cabling. Galaxy Tab 10.1
System Photos - http://s1051.photobucket.com/user/robinkidderminster/library/?sort=3&page=1
Base trap Project - http://www.whathifi.com/forum/home-cinema/corner-base-trap-completed-project?page=1
look after your CD's and none of this BS ever becomes an issue?
Look. Digital data is just that. If it is in tact it doesn't matter if it's read off a sterile aluminium spinning disk, solid state memory or a friggin' squashed marshmallow.
My £10 PC CD reader can rip a disc, in ONE PASS at 40x speed with NO ERRORS! NONE. ZIP. ZERO ERRORS. Sorry to shout but some ppl just aren't getting the memo.
Now. If anyones CD player is having trouble with a clean disc, that it needs to keep applying all this error correction, I suggest you take it back, because it's not working to spec.
Exactly. Buy the CD. Unwrap the CD. Rip the CD. Put the CD back in it's case and store in a cool, dry, safe place. It can't get damaged. (Don't forget to back up the music library now and then.)
In fact "Stand upright in a cool place" is not a bad code for life. (And for bleach bottles.)
Well no, not BS actually.
There may well be no errors on the output of the drive after the error correction has worked its magic, but no physical medium is 100% error free. It is extremely unlikely that a CD or CDROM is 100% bit perfect. Same is true of hard drives, its not that the physical medium is 100%, it is the smart error correction encoding that is used that allows the bit errors to be corrected.
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