Cores in channel for packet voice

The voice-over-packet (VoP) market continues to grow and evolve. From the residential gateway up to central office processing, the market is flooded with multiple standards for codecs, echo cancellation and overall voice channel processing. Channel densitythe number of voice channels that can be handled by a given solutionis a critical issue for evaluating a DSP solution for products. However, there is significant confusion, often intended, in determining real versus theoretical channel density on a given DSP core.

VoP systems are built to process multiple channels of full-duplex digital voice that flow to and from a packet-based network. The two most important signal-processing blocks include voice codecs and echo cancellation devices.

A wide variety of voice codecs is employed in VoP systems, all of which serve to reduce the amount of bandwidth necessary to convey the voice data. Not all voice codecs are created equal. There is a wide difference in their complexity and quality. The International Telecommunications Union (ITU) standardizes many of the codecs used in VoP systems.

Because of the increase in round-trip delay for voice to travel through a packet-based network versus circuit-switched network, it is necessary to reduce the residual echo generated in the local loop from entering the packet network. Most VoP systems employ an implementation of the ITU G.168 standard for digital echo cancellation. The amount of processing power necessary to perform the echo cancellation is tied directly to the length of the tail in the echo canceler's adaptive filter. The length necessary is a function of the distance between the VoP system and the phone in the local loop.

Beyond voice codecs and echo cancelers, a number of other signal-processing functions must be run in the two-way voice channel. These include:

• Tone generation and detection

• Voice-activity detection

• Caller ID generation

• Configuration and control

A key requirement of this framework is the ability to run different codecs and to independently report detection events and generate tones on independent channels.

The DSP necessity

In determining the suitability of a DSP core for two-way voice channel processing, you will want to understand how many MAC operations can be sustained per cycle. Designers should understand that the DSP core should have low-overhead, hardware-assisted capabilities for circular and indexed addressing. This is especially crucial in producing an efficient multi-channel framework.

Because the DSP core is best suited to run processing algorithms of two-way voice channels, designers would rather not have it involved in moving the multi-channel voice data to and from the packet and telephone networks. On the telephone network side of the system, it is effective to have a custom hardware block that can manage the shuttling of voice data to and from buffers in the DSP core memory space and a TDM bus. On the packet network side of the system, it is effective to have the DSP core work in conjunction with a smart direct memory access controller that is capable of shuttling voice packets to and from the DSP core and the network.

If importing the applications source code (or intellectual property) from the DSP vendor, scrutinize the performance data for the applications running on the DSP.

Performance assessment

The performance of a VoP system and the channel density that it can support are not just functions of a particular algorithm's performance on a DSP core. In fact, the benchmarking of a particular algorithm used in the two-way voice channel on a DSP core without reference to the system solution can be misleading. We have found that it is much more accurate to benchmark a VoP system using typical multi-channel system scenarios as a gauge of performance.

Channel density in a VoP system is not a function of algorithm performance on a particular DSP core alone. At the system level, the movement of channel state information and the orchestration of multiple signal-processing functions in the two-way voice channel are key. System-level benchmarking in a multi-channel environment provides a more accurate view of performance and helps designers choose the best VoP solution.

Lloyd Palum

Platform Architect

Barry Gravante

Senior Engineer

Bob Bell

Applications Marketing Director

Improv Systems Inc.