The Story Of Naim's Bit-Perfect CD Ripper
Article By Steve Harris, Software Engineering Manager Of Naim Audio

  All hard disk music players adhere to the well known computer maxim of "garbage in – garbage out". That is why the quality of the CD rip is crucial in obtaining the best performance from such a device. The Naim CD loader was selected specifically for the quality of rip it produces... as well as the lack of noise it generates during the ripping process. We then needed to use bespoke firmware to ensure perfect bit for bit ripping.

There are many issues that arise when you attempt to make quality copies of audio from a CD onto a hard drive. During the development of our Hard Disk Player and Server range, it quickly became apparent that there was a large variation in systems for extracting audio data from CDs.

These issues come down to: Secure Mode ripping and techniques to get the audio samples off the disc; dealing with copy protected and non compliant CDs; drive offset correction; capturing track lead-in and lead-out; and the CD mechanism and the firmware running inside it.

Burst Mode Versus Secure Mode Ripping
Using PCs (or Macs) a CD can be copied to a hard drive within a couple of minutes. Unfortunately, because of the weak error correction system used on audio CDs, the resultant audio can be less than bit perfect. The problems can range from subtle but audible unnecessary interpolation to random clicks and pops when copying used CDs. This type of ripping is known as "Burst Mode" and it assumes that the sectors being read from the disc are perfect; hence a fast copy can be achieved. Some mechanisms provide hardware-assisted correction in this mode, but typically it results in unnecessary interpolation of the audio rather than using techniques to extract the real audio samples from the disc.

The Naim Audio hard disk player and servers use a superior technique called Secure Mode Ripping on a specially selected audio grade CD mechanism. Secure Mode works by using a CD mechanism that does not cache data and reads the sectors several times. It also uses the uncorrectable C2 error feedback information supported by the CD mechanism, so known problematic areas can be re-read and handled accordingly.

The Naim engine typically spins discs at peak speeds of 16-times normal. However, the speed is reduced when problematic areas of the disc are reached. This can help with reading the data accurately off the disc. Given the way the ripping works, where, at a bare minimum, the data is read twice, this reduces the ripping speed to 8-times. Tracks at the start of disc rip slower than those on the outer edge, as the physical spin speed needs to be inherently faster nearer the centre and mechanical speed limits are reached. Also there is overhead locking into the start of each track. So the 'real world' speed is around 7.5-times.

Copy protected discs that contain deliberately corrupted error correction information are detected and the interpolation algorithm as used by a traditional CD player to play such CDs is used, and so the resultant audio data is reconstructed. Typically the Naim player/server will rip a CD in about eight minutes, which is approximately three times slower than burst mode, but gives a far higher level of confidence that the extraction of data off the disc was done properly.

Copy Protected CDs
One of the key requirements of a CD ripping engine is to ensure that it can handle a wide range of real world CDs. Nowadays there are plenty of non standard CDs due to copy protections schemes implemented that break compliance with the original Philips Red book (audio) and Orange Book (audio + data) CD specifications.

Copy protection schemes use four techniques:

Corrupt Table Of Contents (TOC).

Orange and Yellow book CDs can have multiple sessions on them. This scheme assumes that an audio CD player will only look at the first TOC entry, while on a computer drive it will read the other TOC entries which are deliberately corrupted.

Corrupt data layer on Orange Book CDs.

These CDs are a hybrid of traditional audio tracks and an optional computer data section. This opens the opportunity for audio CDs to contain multi-media content. Some copy protection schemes use the quirk that if a PC CD-ROM drive attempts to read a data layer that has unreadable sections on it, then it will give up mounting the disc, so the audio section of the disc is inaccessible to the computer.

Corrupt C1 and C2 Reed-Solomon Information in the audio information.

The copy protection scheme deliberately breaks the error correction information embedded in the audio and forces the player to interpolate. Many ripping systems cannot handle this situation, so they decide that the disc is faulty and abort the ripping process. This type of schemes is especially bad as it makes discs vulnerable to wear and tear and it also affects sound quality.

Auto-run computer software supplied on data layer of Orange Book disc that blocks access to audio tracks.

This is a sinister scheme because it works by surreptitiously installing software on the user's computer that blocks access to the audio tracks on the CD. Some of these small applications – known as Root Kits – were hidden deep in the operating system of the computer and were very difficult to remove. Luckily these schemes were short-lived because the system was 'virus like' in the way it achieved its protection method.

Offset Correction  

Most computer CD-ROM mechanisms suffer from a symptom where there is a small positional error when playing audio CDs. This is because the calculated position on the disc compared to the actual position to which the laser moves is slightly different. The consequence of this is that either the start or end of the track is not captured correctly.

Schemes like AccurateRip have been created that allows users to calibrate their 'data grade' mechanism through their ripping application, by inserting a CD that is known to the AccurateRip database on the Internet. A positive or negative offset can be then calculated to adjust for this offset error. So far, few ripping applications actually support this and it can be hit and miss for the CD mechanism to support this as well (each mechanism must be manufactured consistently), as the finer details of the firmware in CD drives are rarely advertised. The Naim ripping system uses a drive designed for reading audio CDs and is factory calibrated to have no offset error. This ensures that the start and end of tracks are accurately captured.

Capturing Track Lead-in and Lead-out times

CDs have a concept of time between tracks that can be defined by the artist. This can range from nothing (one track merges into the next) to several seconds.

MP3s and WAV files have no concept of this inter-track album spacing, which means that ripped albums typically lose this information. It is up to the player to add a predefined gap between tracks. On gapless albums, such as live concerts, this can ruin the flow of the recording. If this information is not captured at rip time, then it is impossible to add this information at a later stage. The Naim ripping engine captures the lead-in and lead-out times and records this as silence into the WAV audio files. When the player application is playing an album, it seamlessly joins each WAV file together, resulting in the recording on the hard disc playing back the way the artist intended it to be heard.

CD Mechanism Requirements

A computer CD-ROM drive typically connects through the IDE, SATA or USB interfaces and uses the ATAPI protocol. This means that much of the code for getting the data off the disc runs on an embedded microprocessor on the CD-ROM mechanism rather than in the ripping program running on the computer.

The requirements for the CD-ROM mechanism used in the Naim HD player/Servers follow: it must be able to handle C1 and C2 error correction and report back non-correctable C2 errors; it must offer good compliance with copy-protected CDs when reading corrupt TOCs and deliberately corrupted error correction information; it should be mechanically quiet. (Too many drives sound like a jet engine when spinning a CD at high speeds.); it must employ no data caching. (Caching causes havoc because a one-off bad read of audio from the disc will keep on returning the same bad data when asked to read the disc again.); it should hide the data layer from Orange Layer CDs, which can cause various compatibility issues for the drive and operating system if strategically corrupted; it must provide calibrated offset tracking so that the start and end of tracks can be captured accurately; it needs to offer good compliance with tracking warped, off-centre discs and discs with poor reflectivity – CD-R, CD-RW and well worn CDs; it requires a clamping mechanism that can cope with dual-layer discs that are out of specification in terms of thickness.

In Conclusion
Although it is possible to create an equivalent ripping system using a computer, CD-ROM drive and ripping software, the process is fraught with technical issues ensuring that the correct combination of software and hardware is used. Spending weeks ripping your CD collection, only to find that the copies are sub-standard is a very disheartening experience.

Manufacturer
Naim Audio
Southampton Road
Salisbury
Wiltshire 
SP1 2LN
United Kingdom

Voice: +44 1722 426600
E-mail: info@naimaudio.com 
Website: www.naimaudio.com