Audio Decoders: Difference between revisions

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The TV sound needs to be decoded in order to receive an Audio Frequency (Sound_AF) signal.
The TV sound needs to be decoded in order to receive an Audio Frequency (Sound_AF) signal.


There are various possibilities
Historically this feature has been built into the tuner tin can, but this
was only feasible for single TV sound standards. No programming was needed to get
sound from the tuner.


* Sound_AF provides by the tuner
As an alternative tuners always had an extra output Sound_IF (Intermediate Frequency), which can be used by sound decoder chips to extract stereo or
Some tuners have mono out /stereo out /radio out
implement support for multiple sound standards.


* Sound_IF provided by the tuner, this needs an extra decoder
For stereo sound (and later Digital sound standards like NICAM) an
external chips (e.g. msp3415 or other) was introduced and needs to be properly supported by the linux driver.


For BTSC stereo also some discrete module (e.g. NE41S) exists on various cards.
** discrete decoding module (e.g. NE41S fpr BTSC stereo)
** simple audio decoder chip (e.g. tda9820)

The next step was the integration of the sound decoder into the PCI chip (e.g. cx25840 and others).
** DSP audio decoder chip (e.g. MSP3415)
*** all DSP audio chips provide Sound_AF
*** some can provide I2S signal
** Audio decoding funtion built into the PCI bridge chip (e.g. cx25840)


== Sound Routing ==
== Sound Routing ==


Another common problem are the multiple sound sources (e.g. Line In, Tuner Sound_AF, Sound-Decoder_AF) which have to be connected to the PCI interface chip.
There are multiple sound sources on the TV card, as there is only one
output, there must be a switch to select from these:
As this chip only had one sound input, an external switch (crossbar) is required,
* Sound_AF from tuner (mono/stereo/radio)
this is commonly a HEF4052 mux (but sound decoders can have also a crossbar).
* Sound_AF from decoder chip
* Line In


There can be these switches (aka mux or crossbars)
⚫
== Sound Output ==
* HEF4052: this selects from 4 inputs
* some DSP audio decoder chips have multiple inputs
* some PCI bridge chips have multiple input


If wired propoerly by the IHV, the I2S can be read directly by the PCI bdrige chip (so you have one less D/A -> D/A conversion, this should result in less noise).
Historically most TV cards had a Line Out for the TV sound. This can be connected directly to speakers/headphones of to Line In of the sound card.


⚫
== Sound Output ==
Some cards had amplifiers in order to drive speakers or headphones directly. Else you need active speakers (which do the amplification themselves).

Newer PCI bridge chips can optionally digitize Sound_AF and deliver this over the PCI bus. You can direct this digital sound to the sound card.


Sound can be output to:
In order for this to work the card IHV needs to connect the chips properly.
* Line out
** Line out connected to sound card
** Line out connected to active speakers
* Speaker or headphone out (some cards had a headphone amplifier)
* A/D digitizer on PCI chips (e.g. btaudio), this can be directly transferred to memory or can be sent to the audio card for output
* also see I2S above

Revision as of 15:23, 29 January 2006

TV Sound Standards

TV-System Sound Carrier [MHz] Sound Modulation Color System Country
B/G 5.5/5.7421875 FM-Stereo PAL Germany
B/G 5.5/5.85 FM-Mono/NICAM PAL Scandinavia,Spain
L 6.5/5.85 AM-Mono/NICAM SECAM-L France
I 6.0/6.552 FM-Mono/NICAM PAL UK
D/K 6.5/6.2578125 D/K1

6.5/6.7421875 D/K2

6.5/5.85 D/K/NICAM

FM-Stereo

-

FM-Mono/NICAM

SECAM-East USSR

-

Hungary

M

M-Korea

4.5

4.5/4.724212

FM-Mono

FM-Stereo

NTSC USA

Korea

Satellite 6.5

7.02/7.20

FM-Mono

FM-Stereo

PAL ASTRA

TV Sound Decoding

The TV sound needs to be decoded in order to receive an Audio Frequency (Sound_AF) signal.

There are various possibilities

  • Sound_AF provides by the tuner
 Some tuners have mono out /stereo out /radio out
  • Sound_IF provided by the tuner, this needs an extra decoder
    • discrete decoding module (e.g. NE41S fpr BTSC stereo)
    • simple audio decoder chip (e.g. tda9820)
    • DSP audio decoder chip (e.g. MSP3415)
      • all DSP audio chips provide Sound_AF
      • some can provide I2S signal
    • Audio decoding funtion built into the PCI bridge chip (e.g. cx25840)

Sound Routing

There are multiple sound sources on the TV card, as there is only one output, there must be a switch to select from these:

  • Sound_AF from tuner (mono/stereo/radio)
  • Sound_AF from decoder chip
  • Line In

There can be these switches (aka mux or crossbars)

  • HEF4052: this selects from 4 inputs
  • some DSP audio decoder chips have multiple inputs
  • some PCI bridge chips have multiple input

If wired propoerly by the IHV, the I2S can be read directly by the PCI bdrige chip (so you have one less D/A -> D/A conversion, this should result in less noise).

Sound Output

Sound can be output to:

  • Line out
    • Line out connected to sound card
    • Line out connected to active speakers
  • Speaker or headphone out (some cards had a headphone amplifier)
  • A/D digitizer on PCI chips (e.g. btaudio), this can be directly transferred to memory or can be sent to the audio card for output
  • also see I2S above
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