User`s manual

AUDIO INTERFACE

The DDF7001 digitizes the receiver audio and can stream it via UDP. Once a TCP/IP

connection is made the controller sends a Stream Audio (0x0028) command to the DDF7001

signaling the DDF7001 as to which port to use to stream the audio. Once this command is

received the DDF7001 will stream the audio when the audio level exceeds the audio squelch

threshold. This threshold can be set via the command interface.

Compression

The audio can be sent compressed or uncompressed depending on the preference of the

programmer. When compressed A-law compression is used. The algorithm for

decompressing the audio is shown below. If uncompressed audio is streamed then a

maximum of 4 connections is allowed. Compressed audio allows up to 8 streaming

connections.

short decode(byte alaw)

{

//Invert every other bit, and the sign bit (0xD5 = 1101 0101)

alaw ^= 0xD5;

//Pull out the value of the sign bit

int sign = alaw & 0x80;

//Pull out and shift over the value of the exponent

int exponent = (alaw & 0x70) >> 4;

//Pull out the four bits of data

int data = alaw & 0x0f;

//Shift the data four bits to the left

data <<= 4;

//Add 8 to put the result in the middle of the range (like adding a

half)

data += 8;

//If the exponent is not 0, then we know the four bits followed a 1,

//and can thus add this implicit 1 with 0x100.

if (exponent != 0)

data += 0x100;

/* Shift the bits to where they need to be: left (exponent - 1) places

* Why (exponent - 1) ?

* 1 2 3 4 5 6 7 8 9 A B C D E F G

* . 7 6 5 4 3 2 1 . . . . . . . . <-- starting bit (based on exponent)

* . . . . . . . Z x x x x 1 0 0 0 <-- our data (Z is 0 only when

exponent is 0)

* We need to move the one under the value of the exponent,

* which means it must move (exponent - 1) times

* It also means shifting is unnecessary if exponent is 0 or 1.

if (exponent > 1)

data <<= (exponent - 1);

return (short)(sign == 0 ? data : -data);

}