There are many audio formats available today which can be confusing to navigate. This article provides an overview of the major audio formats, their differences, benefits, and use cases to help you make informed choices.
Linear PCM vs Pulse Density Modulation
There are two main ways of storing audio digitally:
- Linear Pulse Code Modulation (Linear PCM): Used on CDs and common downloadable formats like WAV, FLAC, ALAC.
- Pulse Density Modulation: Known by the brand name Direct Stream Digital (DSD). Used on Super Audio CDs (SACDs) and some downloadable formats like DSF, DFF.
Other modulation methods exist but are not commonly used for music distribution. Let’s explore these two main methods further.
Understanding DSD and SACD
DSD uses a technique called delta-sigma modulation and oversampling to capture audio signals at very high sampling rates. This allows the storage of high frequency information beyond human hearing range to achieve better sound quality.
The popular DSD format used is DSD64, indicating a sampling rate 64 times that of CD audio. DSD64 is used on SACD discs. The high sampling rate is achievable because DSD uses only 1 bit per sample. Many consider DSD64 quality comparable to 24-bit/352.8kHz PCM.
There are also DSD128 and DSD256 variants at even higher sampling rates but content availability is limited. In theory, higher sampling rates enable more precise signal capture up to the limits of human hearing. However, higher rates do not always guarantee better sound quality in practice.
DSD adoption has been limited compared to PCM. SACD players are required for disc playback and SACD ripping is challenging. However, DSD downloads can be played on many modern streamers and computer audio setups.
Demystifying PCM and Common Formats
PCM is the format used on audio CDs. It utilizes 44.1kHz sampling rate and 16 bits per sample. To store PCM digitally without compression, the WAV file format is used. However, WAV files are large relative to the audio information contained.
For this reason, lossless compression formats like FLAC and Apple Lossless (ALAC) were created. These can reduce WAV file size by 40-60% without losing any audio data, similar to ZIP files for general data compression. On playback, decompression has to be fast enough to maintain perfect audio quality, which is not an issue with modern hardware.
Beyond CD quality, higher resolution PCM formats also exist:
- 88.2/96kHz 24-bit: Available on DVD-Audio, Pure Audio Blu-ray, and as audio file downloads
- 176.4/192kHz 24-bit: As above
- 352.8/384kHz 24-bit: Only available as audio file downloads currently
Understanding Lossy Compression
For internet transfers and portable use, more aggressive compression was needed to make file sizes manageable given limited bandwidth and storage capacity. This led to lossy formats like MP3 and AAC which save space by removing audio data deemed inaudible to most listeners:
- MP3: The first wildly popular lossy format thanks to file sharing. Tends to need higher bitrates for transparency.
- AAC: More advanced successor to MP3 developed by MPEG. Apple uses their custom AAC variant to achieve better quality at same bitrates.
Typical bitrates providing decent quality:
- AAC 256kbps: Considered transparent to CD by many listeners using basic gear. Useful for portable.
- MP3 256kbps: Detectable loss of quality compared to AAC 256kbps.
- MP3 128kbps+: Significant loss of audio data audible to most listeners.
Choosing Formats for Purpose
With an understanding of key audio formats, you can now match your use case:
- Mobile use: AAC 256kbps provides an excellent combination of sound quality and portability.
- Home listening: Lossless formats like FLAC, Apple Lossless avoid compromising audio quality while saving storage space versus WAV. MQA builds on this with further innovations for best results.
- Archival: WAV remains a safe choice for perfectly preserving original CD audio data. However, equivalent lossless compression can save >50% filesize with no effective quality loss.
Converting Existing Music Libraries
If you want to convert existing CD/WAV music libraries into more efficient lossless formats or create lower bitrate files for portable use, here are common software options:
- dBpoweramp: Extremely versatile tool for ripping, converting between formats, metadata management.
- XLD (Mac): Open source program focused specifically on lossless conversion. Integrates well into workflow.
- ffmpeg: Powerful command line utility capable of batch format conversion once configured.
Stay Tuned for More Insights
We hope this breakdown has helped eliminate some confusion regarding core audio formats. Lossless compression and recent formats like MQA unlock new levels of quality and convenience.