1.3. Audio Processing
Switching from stereo to mono to limit the output noise is automatically performed by MPX decoder. The blending
process from stereo to mono occurs when signal level reaches the threshold which is defined in register bits
CSR0_BLNDADJUST[1:0] +CSR0_BLNDOFS[3:0]. Stereo/mono status is shown in the register bit ST. Mono
operation can be forced by register bit CSR0_MONO.
A 19 kHz pilot notch filter is implemented to eliminate the distortion from 19 kHz pilot tone for some audio analyzer.
High-frequency interferences and noise can be reduced by pre-emphasis and de-emphasis technology to improve SNR.
Since the high audio frequency is accentuated by pre-emphasis universally when FM signal is transmitted, the received
FM signal is applied to attenuate high audio frequency using a de-emphasis filter. The CSR0_DEEM bit can be
programmed for either 50 or 75us for de-emphasis time constant.
The audio output may be muted by CSR0_DIS_MUTE bit. Volume can be adjusted digitally through
Audio output can be attenuated and audible noise can be minimized during weak-signal level by the software mute
function. Four levels of soft-mute tuning (fastest, fast, slow and slowest) are defined in register bits
CSR0_SMUTERATE[1:0]. In addition, 4 levels of attenuation (16,14,12 and 10dB) are defined in register bits
CSR0_SMUTEATT[1:0]. The soft-mute function can be disabled by register CSR0_DIS_SMUTE.
1.4. Tuning and Seeking
The tuning frequency is defined as:
Frequency (MHz) = Channel Spacing (200kHz or 100kHz or 50kHz) * Channel Number + Band Selection (87.5MHz,
76MHz or 64MHz)
The Channel Spacing is selected by CSR0_CHSPACE[1:0]. The Channel Number is defined by CSR0_CH[9:0]. The
CSR0_BAND is used for Band Selection such as Japan, Europe/U.S./Asia or China band. Setting CSR0_TUNE bit is
to enable tuning operation. The seek/tune done (STD) bit set when tuning process is done and RSSI level is available.
When seeking process is initiated by setting CSR0_SEEK, the tuning frequency will be seek up or seek down
depending on the setting of CSR0_SEEKUP. The RSSI and tuned channel are readable in RSSI[7:0] and
READCH[9:0] respectively when the STD bit is set to high to indicate the completion of seek operation. The SF bit is
set to high to indicate that the tuned channel is seek failure when the number of RSSI of all seeking channel is less than
or equal to the threshold number defined in CSR0_SEEKRSSITH[7:0]. Seek channel can be read by READCH[9:0]
during the entire seek operation.
The seek operation can always be deactivated by setting CSR0_SEEK bit to low.
Specifications subject to change without notice