RCA out jack versus headphone jack

[alistair]

Hi-

Is there any reason to prefer the RCA jacks to the headphone out jack? I've been reading that Alsamixer mastervolume will work with the 3.5mm jack, but not the RCA jacks. Since we're retrofitting the card to existing software written with master volume controls, is there any reason we shouldn't just use the 3.5mm headphone jack as the lineout?

Thanks,
Alistair

P.S. I realize that softvol will allow for command line control of the RCA volume but I think we'll run into name conflicts and have to rewrite some of our code.

[flatmax]

The headphone jacks have a large power output, if that doesn't effect your use case then feel free to use the HP jack output. If there is too much power output over the headphone jack for your use case, then you should use the RCA output soft volume (viewtopic.php?f=5&t=572&p=1580#p1580).

You mention name conflicts w.r.t. your code for controlling volumes, you can re-name the volume control almost as you wish using the .asoundrc, hopefully this will save you from re-writing your code. Alternatively couldn't you have the alsa mixer control names as variables in your code ?

thanks
Matt

[alistair]

Hi Matt-

how large is the power output for the headphone jack in comparison to the RCA jacks? We're running a Piscreen so are a little concerned about dropped voltages and brownouts....

thanks,
Alistair

[flatmax]

hi there,

The power output through the headphone depends on the input resistance it sees. For 16 Ohm input resistances, it will only draw 50 mW. That is around 10 mA of current - a very small amount of the total 500 mA which should be available to the Pi from a computer power jack.

The line (RCA) output power consumed would be even less then this figure.

Matt

[alistair]

Thanks Matt.

One more question - if I'm using the RCA jacks and the Pi is off will music passively pass through? I actually don't want this as I'm using the Pi as an active volume attenuator. If the Pi accidentally loses power I want the music to stop. Any advice?

Alistair

[flatmax]

I think you will find that if the pi is off, the sound card's codec is powered down and no audio will pass.

Matt