IP audio Gain adjustment

(New from DSP SW 01.11 / AMCD 10.31)

Audio levels and gain on AlphaCom IP audio connections may be changed using NVRAM programming. Defaults has been set, so that normal usage should work without any changes.

Background

Normal audio is encoded in AlphaCom internal digital audio format (SigmaDelta) with a fairly low level, compared to the max theoretical amplitude of the audio format. A headroom of factor 10 = 20dB is common.

For IP audio connections between AlphaComs, the SigmaDelta audio is converted to a standard format like G.722 or G.711. The gain is 1 = 0dB, in the sense that max SigmaDelta amplitude corresponds to max amplitude on the external format. The audio level on these connection will therefore be quite low.

In the telecom world, the standard audio level is much higher, headroom as small as 3dB is normal. Therefore AlphaCom audio needs to be amplified when sent to external SIP equipment.

Gain adjustment and limiter

The default behaviour is now:

• Inter-AlphaCom and to IP stations: 0dB gain in and out (ie low audio level)
• Towards SIP: +18dB gain out towards SIP, -9db reduction for incomming audio.

Also there is a limiter function. The limiter reduces the effective gain automatically if the signal after gain amplifier exceeds the limit for the connection.

The default limit is -3dB for outgoing audio, and -15db for audio incomming to backplane.

If this works fine, no need to read further.

The gain and limit can be programmed per node in NVRAM. No AlphaPro support is implemented so far. The NVRAM values can be used for testing. AlphaPro support will be added if there is a demand for it.

The NVRAM fields are:

module_profile.node_profile[<node-no>].audio_gain_in 
module_profile.node_profile[<node-no>].audio_gain_out
module_profile.node_profile[<node-no>].audio_limit_in
module_profile.node_profile[<node-no>].audio_limit_out

<node-no>: Selects the Alphacom or SIP device the setting applies to. For SIP, both registrar and trunk node number needs programming. <node-no> = own node controll the gain towards all local IP stations.

Values: 0 (zero) means use default values as described above. Non-zero value: the specified value is used for the matching connections.