Page 3
4.0 Multi-channel Splitting Process
This section outlines the sequence of steps involved in the Splitting Process.
Configure the AAC or AAC-LD Encoder output bit rate to match the capacity of the physical transmission channels minus the overhead bit rate to support the Telos Systems frame header.
Configure the Splitting Process to parse for AAC or AAC-LD encoded audio.
The Splitting Process monitors the encoded audio stream and confirms that it matches the format it expects. The Splitter does not start any data transmission over the physical channels until it detects three frame headers in a row of the expected encoded audio format.
To minimize audio throughput delay, the first two parsed frames are discarded and data transmission starts with the third parsed frame. (It was deemed that there is no useful audio content within the first second of establishing an audio connection with the receiving end, thus the data can be discarded and the third frame which is being received from the Encoder in real-time can be transmitted almost immediately). There is a tradeoff, however, between minimizing the audio throughput delay versus buffering a sufficient number of bits to prevent running out of data to transmit. Because the AAC and AAC-LD frames have variable length, there is the potential that the Splitting Process could run out of data while transmitting the third frame before the next frame arrives. Actually, this could statistically occur between any two consecutive frames. As a result, the Splitter waits until it receives a fixed number of bits beyond detecting the third frame header before it starts data transmission.
Once the Splitting Process has detected the third AAC or AAC-LD frame, it begins the process of dividing the encoded audio stream into multiple, lower bit rate streams based on the number of physical channels that are used to transmit the data. For example, let's assume that the encoded audio is going to be sent across two ISDN B channels at 64 kbps per channel. The Splitting Process allocates memory space for two buffers, one buffer for each B channel. The first 24-bit value written into each buffer is the Telos Systems AAC frame header with the frame count field value set equal to zero. (Refer to Figure 3). Next the AAC or AAC-LD frame header value for the third frame received from the Encoder is stored in the buffer designated as Channel #0 in the Telos Systems frame header. The next consecutive 24 bits of the original encoded audio stream are then copied into the other buffer (designated as Channel #1).
The process of copying every other consecutive set of 24 bits to each buffer continues until sixty-three 24-bit values of the original encoded audio stream has been copied into each buffer. At this point the frame count value is incremented and the next Telos Systems specific frame header value is stored in each respective output buffer. (In general, this step is concurrent with receiving new data from the Encoder and transmitting data over the physical channel). The process of inserting a Telos Systems specific frame header after every sixty-three 24-bit values is the same for 56 Kbps or 64 Kbps physical channels.
Once the Splitter has parsed the third consecutive AAC or AAC-LD frame header, and the necessary number of words has been buffered up to prevent drop outs, the Splitting Process begins transmission over each physical channel. For a 64 Kbps capacity channel, eight bits at a time are taken out of each buffer and transmitted over each corresponding physical channel (most significant bit first). For a 56 Kbps capacity channel, seven bits at a time are taken out of each buffer.
• 
