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 ALC662:Low volume and surround on PC-Headphones with fit-PC2 
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Joined: Tue Mar 09, 2010 9:35 am
Posts: 9
Post ALC662:Low volume and surround on PC-Headphones with fit-PC2
We use fit-PC2 for a long time already and we are grateful for this small and reliable device. We use Linux as OS for the fit-PC2.

In the meantime fit-PC2 rev 1.4 has been released and also offers new features regarding audio: Rev 1.4 comes with sound chip ALC662 which offers 6-channel audio and allows digital 5.1 output which is a great enhancement.

However, everything can be made even better and there are also a few drawbacks of the current ALC 662 audio hardware routing in fit-PC2 with regard to the old ALC260 audio:
  • The analog headphones/line-out output of (old ALC260 and new ALC662 based) fit-PC2 is (hardware) routed from Pin 39 (left) and 41 (right) of the audio chip.
    • Pin 39 and 41 of the old ALC260 chip is called HP-OUT and this output has a headphone amplifier with an output impedance of 1 Ω. At maximum mixer level settings this output achieves an non-clipped signal amplitude of about 1.2 Vrms with a high-impedance load. Using a 32-Ω load (Sennheiser Headset PC 141) the signal does not drop noticeable and does not clip under load -> about 45 mW rms power can be delivered to 32-Ω headphones with the ALC260-based fit-PC2.
    • Pin 39 and 41 of the newer ALC662 chip is called SURR (Port-A) and this output has no headphone amplifier and the output impedance is 100 Ω.
      1. At maximum mixer level settings this output achieves an slightly-clipped signal amplitude of about 1.2 Vrms with a high-impedance load. Using a 32-Ω load (Sennheiser Headset PC 141) the voltage drops noticeable to 0.19 Vrms and the 1-kHz signal is strongly clipped. The output mixer level has to be decreased by -3 dB to get an maximum non-clipped signal of 0.17 Vrms -> only about 0.9 mW rms power can be delivered to the same headphones with the ALC662-based fit-PC2 rev 1.4. This about 17 dB lower maximum audio level with the same headphones (which might be insufficient in a loud environment as in our case).
      2. The second less disturbing issue is that the output of the surround channel "SURR" is sent to the headphones jack if using a standard ALC662 driver.

Issue 1. (too low output power with 32-Ω headphones) is probably the reason for thread low audio volume and is a real problem for us.

Issue 2. (surround channel L&R are routed to headset output) can be worked around by either (software-)rerouting the front output to SURR in a specially tailored driver (most probably done in the Windows driver mentioned in thread No sound from Stereo Line-out (solved)) or at application level by directly sending the stereo output to the surround channel instead of the front channel (as we do it under Linux).

There is a really nice (hardware) solution to the above mentioned issues:
Reroute the headphones/line-out jack wires to Pin 35 (left) and 36 (right) of the audio chip. This is my/our biggest wish for the next fit-PC2 hardware revision!

The above hardware solution (change)
  1. solves problem 1. Pin 35 and 36 of ALC662 is called FRONT (Port-D) and this output has also a headphone amplifier with an output impedance of 1 Ω.
  2. solves problem 2. By default the front channel L&R are routed to this output. Standard driver for all OSs (Windows XP/Vista/7, Linux) can be used and will work out of the box as expected. No development/update/maintenance/setup of special drivers/patches is necessary.
  3. offers a great new feature -> fit-PC2 would become capable of complete 6-channel analog output via its 3 jacks:
    • headset jack gets Front channel L&R by default
    • line-in jack (which is hardware routed from pins 23 and 24 called LINE1) can be switched to Surround L&R output by software
    • microphone jack (which is hardware routed from pins 21 and 22 called MIC1) can be switched to Center&LFE output by software
    This allows playback of 5.1 sound via all these existing analog 5.1 audio sets out there. This is not possible with hardware rev 1.4 (and before).
  4. is backward compatible to the old ALC260. Pin 35 and 36 of this chip is called LINE-OUT and this output has also a headphone amplifier with an output impedance of 1 Ω. By default it get's the same stereo output signals as HP-OUT. So no need to keep an extra design for the old ALC260 chip.
  5. PCB layout change is not too complex, pin 35 and 36 are currently not used and they are located near the old pins 39 and 41. Even an experienced soldering amateur could apply an appropriate hardware wiring "patch" to the existing rev 1.4 (if he/she is willing to sacrifice the warranty for this feature). I could supply instructions on request.

Please forward to the development team and comment.

Best regards,

Wolfgang.


Wed May 11, 2011 6:04 pm
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Joined: Mon Mar 16, 2009 2:11 pm
Posts: 2347
Post Re: ALC662:Low volume and surround on PC-Headphones with fit
Dear Wolfgang,

Thank you for this comprehensive and insightful analysis. I forwarded it to the design team.
There is no plan for another HW revision for fit-PC2 but this feedback will be considered in the audio design of future products.

_________________
-------------------------
Irad Stavi
fit-PC forum admin

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Fri May 13, 2011 9:45 am
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Joined: Tue Mar 09, 2010 9:35 am
Posts: 9
Post Re: ALC662:Low volume and surround on PC-Headphones with fit
Dear Irad,

as I saw on http://www.fit-pc.com/wiki/index.php/Fit-PC2_Revisions that after HW revision 1.4 two more HW revisions have been released (1.41 in July 2012 and 1.51 in November 2014).

Because you forwarded my proposal to the design team already in May 2011, chances are that they took it into account.

Do you know if my proposed change (rerouting of the headphones/line-out jack wires to Pin 35 and 36 of the audio chip) has been incorporated into one of these HW revisions?

Best regards,

Wolfgang


Wed Jul 01, 2015 2:38 pm
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Post Re: ALC662:Low volume and surround on PC-Headphones with fit
we're checking ...


Thu Jul 09, 2015 8:17 am
Post Re: ALC662:Low volume and surround on PC-Headphones with fit
Pins 35, 36 in Audio Codec circuit are not connected and Headphone output routed to pins 39, 41.


Sun Aug 09, 2015 11:35 am
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