Editing N900 FM radio receiver

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The [[Nokia N900|N900]] FM receiver allows the reception of broadcast FM radio signals using the headset/headphones as an antenna.
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The N900 FM receiver allows the reception of broadcast FM radio signals using the headset/headphones as an antenna.
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==Hardware==
==Hardware==
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The FM radio is part of the bluetooth chip.  
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The FM radio is part of the [[N900 Hardware Bluetooth|Bluetooth chip]]. It receives FM radio signals using the headset or earphones plugged into the 3.5 mm socket as an antenna (actually arbitrary headset/headphones, or any other cable, for this purpose).
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It receives FM radio signals using the headset or earphones plugged into the 3.5mm socket as an antenna.
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The audio output of the FM radio is connected to the audio codec chip.
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This can be routed directly to the headphones - but possibly not the speaker due to concerns about speaker damage due to the speakers not being able to handle bass.
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The existing software that uses the radio passes the audio through the CPU, implementing a software filter for speaker protection when the speakers are enabled.
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==Software==
==Software==
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===Kernel===
 
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The FM receiver is implemented as a video4linux radio device by the [http://mxr.maemo.org/fremantle/source/kernel/drivers/media/radio/radio-bcm2048.c bcm2048] module. This controls the receiver over the [[N900 Hardware Bus I2C]].
 
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It appears as device <code>/dev/radio1</code>.
 
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<pre>
 
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v4l2-ctl --all -d /dev/radio1
 
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Driver Info:
 
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Driver name  : bcm2048
 
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Card type    : Broadcom bcm2048 FM Radio Recei
 
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Bus info      : I2C: 0x22
 
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Driver version: 1
 
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Capabilities  : 0x00050400
 
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  Tuner
 
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  Radio
 
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Video input : 0
 
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Frequency: 1518400 (94.900000 MHz)
 
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Video Standard = 0x00000000
 
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Streaming Parameters Video Capture:
 
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Frames per second: 25.000 (25/1)
 
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Read buffers    : 0
 
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Tuner:
 
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Name                : FM Receiver
 
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Capabilities        : 62.5 Hz stereo
 
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Frequency range      : 87.5 MHz - 108.0 MHz
 
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Signal strength/AFC  : 0%/0
 
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Current audio mode  : stereo
 
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Available subchannels: stereo
 
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</pre>
 
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The module is only loaded when the radio is used.
 
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It is also exposed under sysfs.
 
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<pre>
 
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/sys/class/i2c-adapter/i2c-3/3-0022# ls
 
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audio_route              fm_rssi                  rds_data
 
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bus                      fm_search_mode_direction  rds_pi
 
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dac_output                fm_search_rssi_threshold  rds_pi_mask
 
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driver                    fm_search_tune_mode      rds_pi_match
 
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fm_af_frequency          modalias                  rds_ps
 
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fm_best_tune_mode        mute                      rds_rt
 
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fm_carrier_error          name                      rds_wline
 
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fm_deemphasis            power                    region
 
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fm_frequency              power_state              region_bottom_frequency
 
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fm_hi_lo_injection        rds                      region_top_frequency
 
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fm_rds_flags              rds_b_block_mask          subsystem
 
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fm_rds_mask              rds_b_block_match        uevent
 
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</pre>
 
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The sysfs interface allows configuration of region (which affects frequency ranges and other parameters).
 
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===User===
 
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There are 3 userspace application that uses the FM receiver:
 
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[http://maemo.org/packages/view/fmradio/ FMRadio],
 
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[http://maemo.org/packages/view/qradio/ QRadio]
 
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and
 
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[http://maemo.org/packages/view/cfmradio/ C FM Radio]
 
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[[Category:N900 Hardware]]
[[Category:N900 Hardware]]

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