How can I control optical out volume with Rockbox?

[haloxt]

I know this is a really weird question, but the reason why I need to change the volume by using Rockbox (if it is possible) is that the soundcard manufacturer told me I can't control volume via their drivers because of legal issues with the chipset manufacturer, and also because I can't change volume on my headphones ^^.

If there's no easy solution like a cfg file, then that's okay. I just wanted to test out my iriver's optical output quality before I get my new external dac/amp which may come in about a week. I love Rockbox btw ^^.

[Multiplex]

As you have discovered the Volume control doesn't work on the digital output but you should be able to apply smaller changes with some of the DSP functions - by which I mean Replay Gain (Pre-Amp) or Equealizer (Precut), but I don't think that either will give you full range (96dB) adjustment.

[haloxt]

Yay changing precut to -24db made it at a listenable volume ^^. Replaygain didn't affect the volume though. I have two more questions though.

1. Does using pre-cut degrade the optical out sound quality? It sounds not so clear going into my sound card optical in and from my sound card into my headphones. It could just be because of my sound card though, it's a $200 headphone sound card but I wouldn't be surprised if their optical in was not well made. The optical cable I got for just this application was also just 6 bucks :p.

2. Anything I should be careful of that might reset my EQ settings? Because if it goes back to 0db my ears are gonna get a soundblast again which happened the last time I tried this set-up even with volume on rockbox and computer set to 0 :p.

[bascule]

2. Anything I should be careful of that might reset my EQ settings?

Save a copy of your config file then cut out all the entries except the pre-cut setting and rename the file fixed.cfg.

This will ensure the setting is reloaded every time your player starts up. Other than that, don't accidentally reset settings

[dreamlayers]

Yay changing precut to -24db made it at a listenable volume ^^. Replaygain didn't affect the volume though. I have two more questions though.

1. Does using pre-cut degrade the optical out sound quality? It sounds not so clear going into my sound card optical in and from my sound card into my headphones. It could just be because of my sound card though, it's a $200 headphone sound card but I wouldn't be surprised if their optical in was not well made. The optical cable I got for just this application was also just 6 bucks :p.

Yes, it does degrade the signal because by decreasing the amplitude you're decreasing the number of bits used.  The full range of a 16 bit signal is 2^16 = 65536.  If you scale that by -24db you get 65536 * (10 ^ -2.4) = 261.  That's close to 8 bits (256).  This is a noticeable decrease in quality, especially in quiet parts (which use very few bits).

It's probably best to get an inline volume control and put it between the headphone jack and headphones.