Omnia HD & AMR An Unhappy Marriage

While there are some complaints circulating forums the Omnia HD will not be improving its AMR-Narrow Band audio codec whilst recording video. It has been reported on the Samsung innovator forum that there are no plans to swap AMR codec for the better AAC codec.

The audio codec used in the most recent firmware release, xxIE4 is the first such example of this, as it continues to us the AMR codec. It should be noted AAC is used for voice recording. The logic in using AMR lies in its CPU demand. Recording HD video puts a heavy draw on the processor. Using a low quality audio codec, should in theory, allow the phone to run fast while recording. The AMR codec is also optimised for voice coding, while AAC is not.

Now if you want to get into the gritty, extrapolate and discuss in the forum.

AMR

Adaptive Multi-Rate (AMR) is a patented audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now widely used in GSM and UMTS. It uses link adaptation to select from one of eight different bit rates based on link conditions.

AMR is also a file format for storing spoken audio using the AMR codec. Many modern mobile telephone handsets will allow you to store short recordings in the AMR format, both open source (see the external links) and commercial programs exist to convert between this and other formats such as MP3, although it should be remembered that AMR is a speech format and is unlikely to give ideal results for other audio. The common filename extension is .amr.

The frames contain 160 samples and are 20 milliseconds long. AMR uses different techniques, such as ACELP, DTX, VAD and CNG. The usage of AMR requires optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. If the radio conditions are bad, source coding is reduced and channel coding is increased. This improves the quality and robustness of the network connection while sacrificing some voice clarity. In the particular case of AMR this improvement is somewhere around S/N = 4-6 dB for usable communication. The new intelligent system allows the network operator to prioritize capacity or quality per base station.

There are a total of 14 modes of the AMR codec, 8 are available in a full rate channel (FR) and 6 on a half rate channel (HR).

AAC

Advanced Audio Cosing (AAC) is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio.It is specified both as Part 7 of the MPEG-2 standard, and Part 3 of the MPEG-4 standard. As such, it can be referred to as MPEG-2 Part 7 and MPEG-4 Part 3 depending on its implementation, however it is most often referred to as MPEG-4 AAC, or AAC for short.

The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bitrate speech coding to high-quality audio coding and music synthesis.

• The MPEG-4 audio coding algorithm family spans the range from low bitrate speech encoding (down to 2 kbit/s) to high-quality audio coding (at 64 kbit/s per channel and higher).
• AAC offers sampling frequencies between 8 kHz and 96 kHz and any number of channels between 1 and 48.
• In contrast to MP3’s hybrid filter bank, AAC uses the modified discrete cosine transform (MDCT) together with the increased window lengths of 1024 points.

AAC encoders can switch dynamically between a single MDCT block of length 1024 points or 8 blocks of 128 points.

• If a signal change or a transient occurs, 8 shorter windows of 128 points each are chosen for their better temporal resolution.
• By default, the longer 1024-point window is otherwise used because the increased frequency resolution allows for a more sophisticated psychoacoustic model, resulting in improved coding efficiency.