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== How to control the FM tuner chip in the N800 ==

The [[Nokia N800|N800]] internet tablet features a tea5761 FM tuner chip for listening
to FM radio. The kernel driver for this chip was written by Nokia and is GPL'd open source. You can look at it by downloading the Maemo kernel source code.

The driver implements the V4L2 radio interface and is thus controlled like a V4L2 device.

=== How to do it in Python ===

By using Python's <code>ioctl</code> function, you can directly talk to the driver, so there is no need for wrapping some C library.

==== Prerequisites ====

Let's first import the required modules:

<source lang="python">
    import os                 # for opening the device files
    import struct             # for packing and unpacking C structs
    from fcntl import ioctl   # for invoking ioctl calls on the device files
</source>

Since we are going to talk to kernel drivers, we have to use weird numbers. C users would just include the appropriate Linux kernel header files, but Python users have to do it by hand. So let's make some defines for kernel level ioctl stuff:

<source lang="python">
    # kernel definitions for ioctl commands
    _IOC_NRBITS   = 8
    _IOC_TYPEBITS = 8
    _IOC_SIZEBITS = 14
    _IOC_DIRBITS  = 2
    
    _IOC_NRSHIFT   = 0
    _IOC_TYPESHIFT = _IOC_NRSHIFT + _IOC_NRBITS
    _IOC_SIZESHIFT = _IOC_TYPESHIFT + _IOC_TYPEBITS
    _IOC_DIRSHIFT  = _IOC_SIZESHIFT + _IOC_SIZEBITS
    
    _IOC_WRITE = 1
    _IOC_READ  = 2
    
    _IOC = lambda d,t,nr,size: (d << _IOC_DIRSHIFT) | (ord(t) << _IOC_TYPESHIFT) | \
        (nr << _IOC_NRSHIFT) | (size << _IOC_SIZESHIFT)
    _IOW  = lambda t,nr,size: _IOC(_IOC_WRITE, t, nr, size)
    _IOR  = lambda t,nr,size: _IOC(_IOC_READ, t, nr, size)
    _IOWR = lambda t,nr,size: _IOC(_IOC_READ | _IOC_WRITE, t, nr, size)
</source>

The functions <code>_IOW</code>, <code>_IOR</code>, and <code>_IOWR</code> (macros in C) are used for write-access, read-access, and read-write-access, respectively. They basically just take some input values and compute a number which can be sent to the device driver. You can think of the number as the signature of the function which you want to call on the driver.

Now we can generate those weird numbers for the V4L2 functions we are
going to invoke on the driver:

<source lang="python">
    # V4L2 stuff for accessing the tuner driver
    _VIDIOC_G_TUNER     = _IOWR('V', 29, 84)
    _VIDIOC_G_AUDIO     = _IOR ('V', 33, 52)
    _VIDIOC_S_AUDIO     = _IOW ('V', 34, 52)
    _VIDIOC_G_FREQUENCY = _IOWR('V', 56, 44)
    _VIDIOC_S_FREQUENCY = _IOW ('V', 57, 44)
    _VIDIOC_G_CTRL      = _IOWR('V', 27, 8)
    _VIDIOC_S_CTRL      = _IOWR('V', 28, 8)

# user-class control IDs defined by V4L2
    _V4L2_CTRL_CLASS_USER = 0x00980000
    _V4L2_CID_BASE        = _V4L2_CTRL_CLASS_USER | 0x900
    _V4L2_CID_AUDIO_MUTE  = _V4L2_CID_BASE + 9
</source>

And last, let's define some numbers for working with the mixer device:

<source lang="python">
    # mixer control constants
    _SOUND_MIXER_FMRADIO         = 0x06
    _SOUND_MIXER_READ            = 0x80044D00
    _SOUND_MIXER_WRITE           = 0xC0044D00
</source>

==== Opening the device ====

You gain access to the radio driver by opening the <code>/dev/radio</code> device. Since you need a system FD instead of a native Python FD, we use <code>os.open</code> instead of the built-in function <code>open</code>:

<source lang="python">
    radio_fd = os.open("/dev/radio", os.O_RDONLY)
</source>

If there's no tuner chip available (for example, if you run this code on the [[Nokia 770|770]] or the [[Nokia N810|N810]]), this line will throw an <code>OSError</code>. Don't forget to catch it.

==== Retrieving information about the FM tuner ====

Now it's time to talk to the FM tuner. We do this by placing an <code>ioctl</code>'' call on the device. <code>ioctl</code> calls consist of a number (those we have defined above), and some argument. The argument is a pointer for passing a C structure to the driver. The driver may read from this structure or write into it.

A C structure can be built in Python with the <code>struct</code> module. Because the following <code>ioctl</code> call only retrieves information from the driver, we may provide an empty structure of the right size, like this:

<source lang="python">
    info = struct.pack("84x")
</source>

Let's place the <code>ioctl</code>'' call, and retrieve the results. For unpacking the structure, we have to provide the correct format string (see <code>help(struct)</code> for details):

<source lang="python">
    data = ioctl(radio_fd, _VIDIOC_G_TUNER, info)
    
    # unpack the C struct
    fields = struct.unpack("L32sLLLLLLll4L", data)    
    tuner_index = fields[0]
    tuner_name = fields[1]
    tuner_type = fields[2]
    capability = fields[3]
    rangelow = fields[4]
    rangehigh = fields[5]
    rxsubchans = fields[6]
    audmode = fields[7]
    signal = fields[8]
    afc = fields[9]
</source>

We should save the <code>tuner_index</code> and <code>tuner_name</code> for later, because this is how we address that particular tuner.

==== Setting the frequency ====

Next, we are going to tune into a radio station. For this, we have to set a frequency on the tuner. This is done by the V4L2 <code>_VIDIOC_G_FREQUENCY</code> <code>ioctl</code> invokation.

But a word of warning first: the driver does not take the frequency in kHz. You have to multiply your value with a constant factor first. The factor is either 0.016 or 16, depending on the device driver. On the N800, it is 16:

<source lang="python">
    FREQ_FACTOR = 16
</source>

We are going to tune into 107.6 MHz now:

<source lang="python">
    freq = 107600 * FREQ_FACTOR
</source>

Set up the C structure to pass to the driver:

<source lang="python">
    data = struct.pack("LLL8L", tuner_index, tuner_type, freq, 0, 0, 0, 0, 0, 0, 0, 0)
</source>

Pass it to the driver:

<source lang="python">
    ioctl(radio_fd, _VIDIOC_S_FREQUENCY, data)
</source>

==== Unmuting the tuner and setting the volume ====

You still do not hear anything from the radio. This is because the tuner is muted and its volume is set to 0. Let's change this:

Unmuting is easy:

<source lang="python">
    data = struct.pack("Ll", _V4L2_CID_AUDIO_MUTE, 0)
    ioctl(radio_fd, _VIDIOC_S_CTRL, data)
</source>

Changing the volume requires us to open the mixer device:

<source lang="python">
    mixer_fd = os.open("/dev/mixer", os.O_RDONLY)
</source>

Now we can set the volume for the left and right speaker:

<source lang="python">
    left_volume = 50
    right_volume = 50
</source>

... and send it to the driver:

<source lang="python">
    data = struct.pack("bb", left_volume, right_volume)
    ioctl(mixer, _SOUND_MIXER_WRITE | _SOUND_MIXER_FMRADIO, data)
</source>

Do not forget to close the mixer device after this:

<source lang="python">
    os.close(mixer_fd)
</source>

Now you can listen to the radio.

==== Cleaning up ====

When you have finished listening to the radio, you should clean up and 
close the device again.

It's important to mute the radio, or you will hear it forever.

<source lang="python">
    data = struct.pack("Ll", _V4L2_CID_AUDIO_MUTE, 1)
    ioctl(radio_fd, _VIDIOC_S_CTRL, data)
</source>

And don't forget to close the device:

<source lang="python">
    os.close(radio_fd)
</source>

=== Code ready for use ===

There is a pure Python module available for controlling the FM tuner. It is based on the information here. You can get it from [http://pyfmradio.garage.maemo.org the PyFMRadio project page].

[[Category:Development]]
[[Category:N8x0]]
[[Category:Python]]

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Editing Programming N800 FM radio receiver

@@ Line 1: / Line 1: @@
+== How to control the FM tuner chip in the N800 ==
 The [[Nokia N800|N800]] internet tablet features a tea5761 FM tuner chip for listening
 to FM radio. The kernel driver for this chip was written by Nokia and is GPL'd open source. You can look at it by downloading the Maemo kernel source code.
@@ Line 4: / Line 6: @@
 The driver implements the V4L2 radio interface and is thus controlled like a V4L2 device.
-== How to do it in Python ==
+=== How to do it in Python ===
 By using Python's <code>ioctl</code> function, you can directly talk to the driver, so there is no need for wrapping some C library.
-=== Prerequisites ===
+==== Prerequisites ====
 Let's first import the required modules:
@@ Line 72: / Line 74: @@
 </source>
-=== Opening the device ===
+==== Opening the device ====
 You gain access to the radio driver by opening the <code>/dev/radio</code> device. Since you need a system FD instead of a native Python FD, we use <code>os.open</code> instead of the built-in function <code>open</code>:
@@ Line 82: / Line 84: @@
 If there's no tuner chip available (for example, if you run this code on the [[Nokia 770|770]] or the [[Nokia N810|N810]]), this line will throw an <code>OSError</code>. Don't forget to catch it.
-=== Retrieving information about the FM tuner ===
+==== Retrieving information about the FM tuner ====
 Now it's time to talk to the FM tuner. We do this by placing an <code>ioctl</code>'' call on the device. <code>ioctl</code> calls consist of a number (those we have defined above), and some argument. The argument is a pointer for passing a C structure to the driver. The driver may read from this structure or write into it.
@@ Line 113: / Line 115: @@
 We should save the <code>tuner_index</code> and <code>tuner_name</code> for later, because this is how we address that particular tuner.
-=== Setting the frequency ===
+==== Setting the frequency ====
 Next, we are going to tune into a radio station. For this, we have to set a frequency on the tuner. This is done by the V4L2 <code>_VIDIOC_G_FREQUENCY</code> <code>ioctl</code> invokation.
@@ Line 141: / Line 143: @@
 </source>
-=== Unmuting the tuner and setting the volume ===
+==== Unmuting the tuner and setting the volume ====
 You still do not hear anything from the radio. This is because the tuner is muted and its volume is set to 0. Let's change this:
@@ Line 180: / Line 182: @@
 Now you can listen to the radio.
-=== Cleaning up ===
+==== Cleaning up ====
 When you have finished listening to the radio, you should clean up and
@@ Line 198: / Line 200: @@
 </source>
-== Code ready for use ==
+=== Code ready for use ===
 There is a pure Python module available for controlling the FM tuner. It is based on the information here. You can get it from [http://pyfmradio.garage.maemo.org the PyFMRadio project page].