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CD6420 Datasheet(PDF) 8 Page - Cirrus Logic |
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CD6420 Datasheet(HTML) 8 Page - Cirrus Logic |
8 / 52 page CS6420 8 DS205PP2 OVERVIEW The CS6420 is a full-duplex speakerphone chip for use in hands-free communications with telephony quality audio. Common applications include speakerphones, inexpensive video-conferencing, and cellular phone car kits. The CS6420 requires very few external components and allows system control through a microcontroller interface. Hands-free communication through a microphone and speaker typically results in acoustic feedback or howling because the loop gain of the system ex- ceeds unity by the time audio amplitudes are ad- justed to a reasonable level. The solution to the howling problem has typically been half-duplex, where either the transmit or the receive channel is active, never both at the same time. This prevents the howling, but diminishes the overall communi- cation quality by clipping words and forcing the talker at each end to wait for the talker at the other end to stop speaking. Full-duplex conversation, where both transmit and receive channels are active simultaneously, is the conversation quality we enjoy when using hand- sets. Full-duplex for hands-free communications is achieved in the CS6420 using a digital signal pro- cessing technique called “Echo Cancellation.” The end result is a more natural conversation than half- duplex, with no awkward breaks and pauses, as if both parties were speaking to each other directly. Echo Cancellation reduces overall loop gain and the acoustic coupling between speaker and micro- phone. This coupling reduction prevents the annoy- ing effect of hearing one’s own delayed speech, the effect being worse when there is delay in the sys- tem, such as vocoder delay in digital cellular phones. The CS6420 is a complete system implementation of a Digital Signal Processor with RAM and pro- gram ROM, running Echo Cancellation algorithms developed at Crystal Semiconductor using custom- er input, integrated with two delta-sigma codecs. The CS6420 is intended to provide a full-duplex speakerphone solution with a minimum of design effort while displacing existing half-duplex speak- erphone chips. FUNCTIONAL DESCRIPTION The CS6420 is roughly divided into four external interface blocks. The analog interfaces connect the chip to the transmit and receive paths. Certain con- trol functions are accessible through the microcon- troller interface. Two pins accommodate either a crystal or an externally applied digital clock signal. Analog and digital power and ground are provided through four pins. Analog Interface In a speakerphone application, one input of the CS6420 connects to the signal from the micro- phone, sometimes called the near-end input or transmit input, and one output connects to the speaker. The output that leads to the speaker is sometimes called the near-end output or receive output. Together, the input and output that connect to the microphone and speaker are referred to as the Acoustic Interface. The signal received at the near-end input is then passed to the far-end output or transmit output after acoustic echo cancellation. This signal is sent to the telephone line. The signal from the telephone line is received at the far-end input, also called the re- ceive input, and this signal is passed to the receive output after network echo cancellation. Together, the far-end input and output form the Network In- terface. The analog interfaces are physically implemented using delta sigma converters running at an output word rate of 8 kHz, resulting in a passband from DC to 4 kHz. Because the inputs are analog to dig- ital converters (ADCs), certain design consider- ations must be kept in mind: specifically, anti- aliasing and full-scale input voltage. The ADCs ex- pect a single-pole RC filter with a corner at 8 kHz, |
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