Most digital audio systems encode analog – the sound we hear – into digital for storage and transmission using a system called Pulse Code Modulation. Two factors determine the quality of a digital recording: bit depth and sampling frequency. Bit depth determines the number of ‘steps’ available to describe the sound: the more bits used, wider the dynamic range – ie the difference between the loudest and softest sound – one can record.
Meanwhile, the higher the sampling frequency – in other words, the number of times a ‘snapshot’ of the sound is taken each second – the more accurately the music can be analysed and turned into digital data. The sampling frequency affects the audio frequency range – from the lowest to highest pitch – able to be stored.
So the greater the bit depth, and the higher the sampling frequency, the more information can be stored.
CD uses 16-bit/44.1kHz encoding, which was as good as was available when CD was launched at the beginning of the 1980s, but things have moved on, and now we can record and distribute music at greater bit-depths and greater sampling rates. These formats have been used in studios and mastering for many years – now they are available for us all to enjoy at home.
High-Resolution Audio (HRA) is any format beyond the 16-bit/44.1kHz CD standard, and HRA recordings normally use 24-bit encoding, allowing a much wider dynamic range than CD, and sampling rates all the way up to 192kHz, which is the current state of the art for commercial HRA recordings. It is all about getting you closer to the studio sound.
High-Resolution Audio formats
There is a range of HRA formats, so it is important that HRA equipment should support as wide a range as possible.
CD-quality music is also available in a number of formats – here is a rundown of some of the common formats used for both CD-quality lossless music and HRA.
FLAC: Studio Master Sound Quality (24-bit) and CD Quality (16-bit)
Most music files (both HRA and CD quality) available online come in FLAC – the Free Lossless Audio Codec (the format is free, not the music!). FLAC 24-bit files are usually available in 96kHz and 192kHz versions, though a few albums come in 24-bit/44.1kHz or 24-bit/48kHz. The 24-bit/192kHz versions are the highest quality files commonly available, and are identical to the studio master.
Unlike MP3, which throws some content away to reduce file sizes, FLAC is lossless, and works like a computer zip file. It's uncompressed 'on the fly' as you play the music, and delivers exactly the same data present before the file was compressed.
Apple has its own lossless: Apple Lossless (ALAC) works like FLAC, but is compatible with iTunes. Some companies offering Studio Master downloads offer them in both FLAC and ALAC. A further benefit of FLAC and ALAC is that they retain information about the music in the form of metadata, covering common parameters such as artist, album title, track title/number, music genre, composer, catalogue number and so on.
This information will be encoded into any Studio Master or other HRA files you buy online, or can be added to CDs you rip: the ripping software will use an Internet database lookup to identify the disc you are storing, and fill in all the information. You can also edit this data, or enter it manually.
WAV/AIFF: Uncompressed storage
You can also rip CDs as completely uncompressed files – i.e., a straight copy of the data on the disc. Windows computers store these as WAV (Waveform Audio File Format) files, Macs as AIFF (Audio Interchange File Format), but the two are interchangeable. One downside with storing WAV files is that they do not store track information by default – that is one reason why using FLAC is much more convenient. The other is that FLAC files are much smaller than WAVs. Some Studio Masters labels do offer music in WAV form as well as in FLAC.
Used as the basis of the Super Audio CD format, Direct Stream Digital is a format using a rather different method of encoding: rather than greater bit-depths, it uses single-bit, but at a much greater sampling frequency, in order to record, store and play extremely high sound quality. As originally developed, it used a sampling rate of 2.822 MHz, and this format is known as DSD64,(64 times the sampling of CD) but in recent times even higher sampling rates have been developed, at DSD128 and DSD265. There is even a DSD512 format now being used in some recording studios.