CD Ripping - Audio Extraction Methods Compared
- — 30 October, 2002 15:15
If you have read our Advanced Guide to CD Ripping, then you should be familiar with the process of CD audio extraction. What you may not know are the various methods that are used to do this and the pros and cons of each. This week we take a look under the hood of CD ripping.
Software referred to in this article:
The truth about ASPI
One of the most confusing aspects of CD ripping is the role played by ASPI drivers, sometimes also referred to as the ASPI layer or ASPI interface. ASPI is an acronym for Advanced SCSI Programming Interface. The term SCSI stands for Small Computer Systems Interface, and refers to the hardware interface used by (usually older) fast hard disk drives. While very few PCs actually contain SCSI hardware, more recent versions of the ASPI interface can still be used to access IDE drives, which are by far the most common interface for CD drives.ASPI provides CD Recording and CD Ripping software with direct access to the CD hardware without having to go "through" the operating system drivers. The original ASPI drivers were released by Adaptec, although these days there are third-party ASPI drivers as well as hacks of Adaptec versions for particular uses.
You can get the latest ASPI drivers with the Adaptec ASPI verification utility.
Windows 2000 and XPWindows 2000 and Windows XP both contain native drivers that can be used instead of ASPI for CD audio extraction. Not all CD ripping software supports the native drivers, however, and those that do often warn you against using them, as they can be unreliable. That said, if you have trouble using ASPI with these operating systems, it may be an alternative for you. EAC and Audiograbber both support the native drivers.
If you are using Windows 95 then you won't be able to use the ASPI drivers to perform CD audio extraction. Nevertheless, you can still rip CDs using the MSCDEX DOS drivers with some software. Audiograbber supports the use of these DOS drivers, which were standard back in 1995, when ASPI was only available for SCSI hardware.
Secure audio extraction is used by EAC (Exact Audio Copy). This technique involves reading every sector of the CD at least twice. If there are discrepancies between the two reads, additional reads will be performed until at least a certain number are identical. This number depends on the level of error checking you select.
Most CD rippers provide you with two types of synchronised extraction and two types of buffered extraction. Fixed synch extraction means that the program reading the CD will extract a set amount of data each time it accesses the disc. With each access, an overlap is used to help prevent any skipping or duplication of data during the extraction process.
Dynamic synch performs overlapping as in fixed synch extraction, but the amount of data per drive access is dynamic in size. This amount of buffering fluctuates as needed. If any desynchronisation occurs, a larger amount of overlap will be used to overcome any possible errors.
In contrast to the two synchronisation methods described above, burst copying does not provide any overlap protection. Instead, the method involves continuous reading of the disc for audio extraction. Audiograbber, for example, uses a three-second buffer. If the software cannot process the three seconds of data before the next "burst" of data, a possible speed error occurs. Possible speed errors indicate that the audio may have skipped but there is no way of determining that it has without listening to the result.
This is the same process as above, but without the buffer. This is by far the most risky extraction method and should only be attempted at very low speeds to prevent errors during extraction.
When all else fails - analog extraction
If you tried all the options and can't get a good result, there is always analog conversion. This should be considered as a last resort, because all other extraction methods are digital - you don't even need a sound card to extract audio digitally. Analog extraction, however, is simply a matter of recording the audio as you hear it from your sound card. The problem with this is that the recording is only as good as your sound card. Even professional sound cards with top notch DAC (Digital to Analog Converters) will result in signal loss due to the conversion process. Consumer sound cards will also introduce noise and other artefacts that will degrade the resulting quality.
To perform analog extraction you can use Winamp, an audio editor such as CoolEdit, or even some CD rippers like Audiograbber that have analog options.