April 19, 2016 Views:362
Now network cameras support the following audio coding standards including PCM\G.711\G.722\G.726.
Pulse code modulation (PCM) is a kind of uncompressed digital audio technology. It has been widely used in CD, DVD and other various occasions that demand highest audio quality.
The principle of PCM in various applications is the same. The only difference is in the sampling frequency and quantitative accuracy. The specification of ordinary CD is 16 bits/44.1kHz. For DVD, the quantitative accuracy can be divided into 16 bits, 20 bits, 24 bits while sampling frequency has 48kHz and 96kHz. In addition, the PCM signal can be added with dolby surround sound information for the use of the existing dolby directional logic surround sound system.
G.711 is a kind of audio coding standard set by the international telecommunication union (ITU - T), mainly using pulse code modulation on the audio sampling and the sampling rate is 8k/s. It uses a 64Kbps uncompressed channel to transfer audio signal with compression ratio of 1:2, namely the 16-bit data compressed to 8-bit data. G.711 is by far the most commonly used waveform voice codec.
There are two compression algorithms under G.711. One is G.711U-law, mainly used in North America and Japan. The other is G.711A-law, suitable for Europe and the rest regions.
Hikvision cameras support both of the algorithms.
(3). MP2L2 (MPEG-2 Audio Layer-2)
MP2L2 is an open coding scheme derived from MPEG-1. MP2L2 audio compression technology is a kind of distortion compression which removes the voice frequency range that human can hardly hear. Adopting efficient compression algorithm, it can get very high compression ratio reaching 12:1, making the file size much smaller.
The frequency of MP2L2 varies from 16kHz to 44kHz. The bit rate of Hikvision audio equipment can be set from 32Kbps to 160Kbps and under the bit rate of 128 Kbps, the sound quality of MP2L2 can reach close to that of CD. But as part of the frequency is removed (high frequency and low frequency), it is not as good as CD.
(4). WMA（Windows Media Audio）
WMA, short for Windows Media Audio, is Microsoft music file format. According to Microsoft statement, compared to WMA 8, the latest version WMA 9.0 has a 20% improvement in the compression ratio.
Similar to MP3, WMA is also a kind of distortion compression, but it still has a lot of advantages.
1) Same audio quality but smaller capacity.
2) Better quality under constant frame rate.
3) Suitable for low speed transmission.
4) More close to the original audio in the spectrum structure, better sound fidelity.
G.722 is a kind of audio coding standard set by the International Telecommunication Union (ITU) in 1988. Its sampling frequency is expanded from 8kHz to 16kHz and voice quality is improved.
Proposed by CCITT on the basis of G.721 and G.723 in 1990, G.726 is finally defined by ITU-T. G.726 can convert 64kbps PCM signal to 40kbps\32kbps\24kbps\16kbps ADPCM signal with smaller bandwidth compared to G.711.