[Gta04-owner] The sound level changes with or without remote USB cable

EdorFaus edorfaus at xepher.net
Mon Nov 12 20:57:08 CET 2012

On 11/11/2012 10:25 PM, Marc Langlois wrote:
> In both cases, I hear my voice (with and without cable).

> I'm a little surprised by this result.

To be honest, I'm not, really. The modem is by far the most complex part 
in the (HW routing) audio chain, and thus the most likely part to fail, 
especially when things get strange like this.

> Note: The signal is partially noisy.

This is pretty much to be expected - arecord defaults to 8bit 8000Hz 
audio, which isn't very good quality at all. It can probably be improved 
a bit by telling it to use 16bit 48kHz (or 44.1kHz) audio, but don't 
expect miracles either - it's not like this is studio equipment.

> Is it possible that the "noise reduction" or "Echo Reducer" cut my
> voice to my corespondant?

I suppose that theoretically, yes, but in practice, not really. It's 
highly unlikely, with your symptoms and what Radek said about there 
being two separate ones used depending on the routing path, as it would 
mean that both would have to fail at the same time, and one of them in a 
very strange way.

> But the main question is:
> What is the difference between a call with or without USB cable ?
> There must hardware and software differences.

When it comes to the default HW-routing audio path (and probably the 
SW-routing one as well), I think there's no software difference between 
those two cases - as there is, as far as I can tell, no reason for there 
to be. (There are software differences, sure, but not related to the sound.)

However, there *is* a significant hardware difference - namely power 
availability. As you said yourself, on battery you get 3.76V, while on 
USB you get 3.84V - and there may be other minor differences too.

If the hardware is already marginal, that 0.1V difference (2%) can 
easily be the difference between working and not working - which would 
probably also explain why it doesn't work using SW routing, in that the 
SoC's CPU has to be active in that case, which also draws power, leaving 
less for the rest.

However, AFAIK the hardware is not supposed to be marginal at that point 
- which means there's probably something wrong with the hardware of your 
phone, like Radek said. I would guess something with the modem (or its 
surrounding components, if any). :(

I'm afraid there's probably no real solution to this, other than 
replacing it - at least not without significant electronics knowledge 
and specialized tools.

I can think of a possible workaround, but it's rather awkward: get hold 
of a USB battery pack, and plug it into the phone whenever you need to 
talk with someone. That is rather cumbersome and fragile (in that you 
have to remember to bring and recharge that external battery pack in 
addition to the phone itself), so it may not be worth it...


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