CD quality: what exactly does that mean?

In the audio field you often come across the term "CD quality". The term is used particularly for audio codecs, but also for sound cards and other audio hardware and software. We explain to you what this term is all about.

CD quality for audio hardware and software

In the case of digital devices, the technical sizes of the audio CD are usually used to justify the term "CD quality". The term "CD transparency" is also used occasionally. The following technical parameters play a role:

• The audio CD has been standardized and has been so widespread since the 1980s that it is often used as a benchmark, i.e. as a benchmark.
• Audio CDs have two PCM channels.
• Each has a sample rate of 44, 100 samples per second, that is 44.1 kHz.
• Each sample is encoded with a 16-bit sample depth. The dynamic range, i.e. the maximum signal-to-noise ratio, is therefore 93 dB.
• For digital devices, CD quality means 44.1 kHz at 16 bits. That is a data rate of 1411.2 kbps.
• Common terms to express superiority over CD quality are "DVD quality" and "studio master". These often describe a sample rate of 48 or 96 kHz with a sample depth of 24 bits.
• With "CD quality" it is also important that all frequencies up to 22050 Hz are transmitted. Many microphones, telephones and loudspeakers fail because of this. You can only record, transmit or play back a narrower frequency range.
• In the case of analog devices, stereo quality means that the frequency and phase response are smooth in the entire audible frequency range and the dynamics are comparable to the accuracy of CD. This is a signal-to-noise ratio of approx. 93 dB and an accuracy of ± 0.5 dB.

CD quality for audio codecs

Extensive listening tests were carried out for lossy audio codecs such as MP3, AAC, AC-3 and ATRAC in order to demonstrate from which compression rate CD quality is achieved:

• With audio CDs, all frequencies from 0 to 22050 Hz are reproduced with a sound pressure level accuracy of ± 0.5 dB. Under ideal conditions, sound pressure level changes of about 0.3 to 1.5 dB are just perceptible.
• CD quality means that all audible frequencies are reproduced without any perceptible amplitude errors or distortions over a dynamic range of up to 93 dB. This is significantly larger than the average dynamic range of music and speech.
• In this case, CD quality refers to hearing perception. The audibility of artefacts, ie noise or distortion, as well as spectral, temporal and dynamic deviations from an original in technical CD quality are compared.
• Spatial aspects such as localizability, the perceived extent of the source or the so-called "enveloping" by spatial sound are not compared.
• Speech codecs such as GSM, CELP and G.711, which are used particularly in telephony, are sufficient for the usual compression rates for good speech intelligibility. But they are far below CD quality because they sometimes sound narrow-band and clearly distorted.
• Also records are not CD quality, as we discuss in detail in this practical tip.
• The MP3 audio codec has CD quality from data rates of approx. 128 kpbs. That is a compression rate of 1:11. Experienced listeners and particularly dynamic music demand higher bit rates.
• AAC and the audio codec provided for the MPEG-H require even lower bit rates in order to still have CD quality.
• Lossless codecs like FLAC and ALAC maintain quality. If the source material was CD-quality, it also has CD-quality.

CDex - Easily convert CDs to MP3

A comprehensive comparison between audio codecs such as MP3, AAC, WMA and OGG can be found in another CHIP online article.