CS8421-CNZ Cirrus Logic Inc, CS8421-CNZ Datasheet - Page 20

• Manufacturer Part Number: CS8421-CNZ
• Description: IC SAMPLE RATE CONVERTER 20QFN
• Manufacturer: Cirrus Logic Inc
• Type: Sample Rate Converterr

Specifications of CS8421-CNZ

• Package / Case: 20-QFN
• Applications: Digital Audio
• Mounting Type: Surface Mount
• Operating Supply Voltage: 3.3 V / 5.0 V
• Operating Temperature Range: - 10 C to + 70 C
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: 598-1018 - BOARD EVAL FOR CS8421
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 598-1730

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4.3
4.3.1
4.3.2
4.3.3
Sample Rate Converter (SRC)
Multirate digital signal processing techniques are used to conceptually upsample the incoming data to a very
high rate and then downsample to the outgoing rate. The internal data path is 32-bits wide even if a lower
bit depth is selected at the output. The filtering is designed so that a full input audio bandwidth of 20 kHz is
preserved if the input sample and output sample rates are greater than or equal to 44.1 kHz. When the out-
put sample rate becomes less than the input sample rate, the input is automatically band-limited to avoid
aliasing products in the output. Careful design ensures minimum ripple and distortion products are added
to the incoming signal. The SRC also determines the ratio between the incoming and outgoing sample rates
and sets the filter corner frequencies appropriately. Any jitter in the incoming signal has little impact on the
dynamic performance of the rate converter and has no influence on the output clock.
Data Resolution and Dither
When using the serial audio input port in Left-Justified and I²S Modes, all input data is treated as 32-bits
wide. Any truncation that has been done prior to the CS8421 to less than 32-bits should have been done
using an appropriate dithering process. If the serial audio input port is in Right-Justified Mode, the input
data will be truncated to the bit depth set by SAIF pin setting. If the SAIF bit depth is set to 16-, 20-, or 24-
bits, and the input data is 32-bits wide, truncation distortion will occur. Similarly, in any serial audio input
port mode, if an inadequate number of bit clocks are entered (i.e. 16 clocks instead of 20 clocks), the input
words will be truncated, causing truncation distortion at low levels. In summary, there is no dithering
mechanism on the input side of the CS8421, and care must be taken to ensure that no truncation occurs.
Dithering is used internally where appropriate inside the SRC block.
The output side of the SRC can be set to 16-, 20-, 24-, or 32-bits. Dithering is applied and is automatically
scaled to the selected output word length. This dither is not correlated between left and right channels.
SRC Locking and Varispeed
The SRC calculates the ratio between the input sample rate and the output sample rate and uses this in-
formation to set up various parameters inside the SRC block. The SRC takes some time to make this cal-
culation, approximately 4200/Fso (87.5 ms at Fso of 48 kHz).
If Fsi is changing, as in a varispeed application, the SRC will track the incoming sample rate. During this
tracking mode, the SRC will still rate convert the audio data, but at increased distortion levels. Once the
incoming sample rate is stable, the SRC will return to normal levels of audio quality. The data buffer in the
SRC can overflow if the input sample rate changes at greater than 10%/sec. There is no provision for
varispeed applications where Fso is changing.
The SRC_UNLOCK pin is used to indicate when the SRC is not locked. When RST is asserted, or if there
is a change in Fsi or Fso, SRC_UNLOCK will be set high. The SRC_UNLOCK pin will continue to be high
until the SRC has reacquired lock and settled, at which point it will transition low. When the
SRC_UNLOCK pin is set low, SDOUT is outputting valid audio data. This can be used to signal a DAC to
unmute its output.
Bypass Mode
When the BYPASS pin is set high, the input data bypasses the sample rate converter and is sent directly
to the serial audio output port. No dithering is performed on the output data. This mode is ideal for passing
non-audio data through without a sample-rate conversion. ILRCK and OLRCK should be the same sam-
ple rate and synchronous in this mode. The group delay in this mode is greatly reduced from normal SRC
mode as noted in the
“Digital Filter Characteristics” on page
12.
CS8421
DS641F5