1 PCM port

• Non-Linear Acoustic Echo Cancellation (NLAEC)
• Clipping Compensation
• ROUT and SOUT Equalizers (EQ)
• Psycho acoustic Noise Reduction (PNR)
• Automatic Level Control (ALC)

The ZLS38500 and ZLS38502 are geared towards high-end hands free applications whereas the ZLS38501 is geared towards lower performance cost sensitive applications.

• AT25F512A
• AT25F1024A
• AT25F2048
• AT25F4096
• M25P05-A
• M25P10-A
• M25P32
• M25P64

If you are planning on using the diagnostics tool to help test and tune your prototype, then you must use the M25P32 or M25P64 flash during the prototype phase.

• Frequency Response: the flatter the frequency response in the voice range the better (30 Hz - 4 kHz for 8 kHz sampling and 30 Hz - 8 kHz for 16 kHz sampling)
  
  
• S/N ratio: the smaller the S/N ratio the better
  
  
• Sensitivity: the more sensitive the better

Q

What sort of speaker should I use?

• Most non-linear artifacts in your system will originate from the acoustic path speaker. If you are designing a desktop phone system with the ZLS38501 firmware, you must pick a speaker and speaker driver which will generate less than 2% THD when driving approx. 83-87 dB at a distance of 50 cm directly in front of the speaker. If you are designing a system using the ZLS38500 or ZLS38502, the 2% THD level is also a good guideline. However, because these firmware have a Non-Linear Echo Canceller, you can exceed the 2% THD on the speaker and speaker driver and the system will still function properly. The drawback will be added distortion on SOUT. When designing for hands free car kits, the same criteria applies except that you should strive to achieve 90+ dB on your speaker output with at a distance of 50 cm directly in front of the speaker
  
  
• Frequency response of the speaker is also an important factor when choosing your speaker. The flatter the frequency response is the better.
  
  
• It is also important to select a driver with adjustable gain for tweaking of the system

Q

At what level should I set the NR?

The public telephone networks create echoes where analog phone lines interface to the network. A 2-to-4 wire hybrid makes this interface in the switching equipment located at the telephone switching office, on-premise PBX, or Key System. Because the interface is not perfect, the voice signals echo at the hybrid. Wireless, Satellite, ATM, Frame Relay, Internet and Trunk systems introduce delay that makes the echo perceivable to the subscriber. Echo cancellers remove this echo.

An echo canceller is a specially designed filter for removing echo created in the telephone network. It performs several tasks: it measures the echo path, adjusts an adaptive filter that estimates and replicates the echo, subtracts the replicated echo from the actual echo, and removes any residual echo by using non-linear processing techniques. These tasks are extremely complex DSP functions.

The ability of the echo canceller to train to the echo and remove it, this takes a finite time measured in ms. Network line echo canceller convergence does not need to be particularly fast as network echo is fairly constant. There are two types of convergence, fast convergence and slow convergence.

TFO stands for Tandem Free Operation. TFO is needed to maintain quality of voice when multiple basestations are connected together in a wireless network. The quality of speech degrades due to multiple encoding and decoding operations in these networks. TFO disable multiple encoding/decoding while the signal is passing through multiple wireless base-stations.

Recent vintage V.32/V.32bis/V.34 modems have their own built-in adaptive filters designed to minimize bit-errors for high speed data transmission. Voice echo cancellers in the network can cause these adaptive filters to incorrectly adapt resulting in increased bit-errors during data transmission. To minimize bit-errors, manufacturers of V.32/V.32bis/V.34 modems expect to disable and bypass voice echo cancellers with a Disable Tone per ITU-T recommendations such as G.165. To support these transmissions, the ZL50211and ZL50232 have Disable Tone signal detection for each channel that allows bypassing of its echo canceller function. It is configurable for either ITU-T G.165 or G.164 recommendations.

Given non-linearities in today's telecommunication systems, it is impossible to completely remove the echo with an echo canceller. A Non-Linear Processor is used to remove this residual echo. The background noise is also removed thereby creating an audible switching effect that is very annoying to the subscriber. Zarlink's Advanced Non-Linear Processor removes the residual echo and injects comfort noise at roughly the same loudness as the original background noise to mask-out this switching effect. The result is an excellent sounding telephone connection with no echo and switching effects.

Please refer to the diagram below. Talker A ís speech and its echo "trains" the echo canceller to adapt and remove the echo. When talker B starts speaking, it interferes with talker A ís echo causing the echo canceller to incorrectly adapt and worsen its performance. To prevent the effects of the interference from talker B, the echo-canceller have Double-Talk detection. The Double-Talk detection determines when talkers A and B are speaking simultaneously and freezes the echo canceller to stay adapted and maintain excellent performance.

Once fully adapted, the echo canceller does a very good job of removing the echo. Narrow-Band signals such as DTMF and Call Progress tones can make the echo canceller incorrectly adapt and significantly worsen its performance. The Narrow-Band detector detects these discrete tones, and freezes the echo canceller to stay adapted and maintain excellent performance.

When an echo canceller removes the echo any background noise is also removed. If the noise is removed at the same time the echo is removed there will be rising and falling of background noise, this is known as switch loss. To the end user this is perceivable.