EQ Advice

[Stev]

Hi guys,

Not sure if this is in the right place, if it's not Llorean please be kind

I have an accessory for my ipod which charges, holds and broadcasts an FM signal from the car cigarette holder. It plugs into the dock but obviously only uses the line-out pin for audio because it works fine with Rockbox.

My problem is this, I'm getting a kind of distortion (that's the best I can describe it) but it doesn't seem to be caused by loudness. The sticker on the accessory says it has a frequency range of 20Hz - 20KHz, so I suspect that the distortion I'm hearing is from sounds that fall outside of that range (I could be completely wrong).

I know that sound quality on FM broadcast is average at best, but if I can avoid this distortion by tweaking the EQ to limit to that frequency range then that would be great. I'm using a 5G 60GB iPod, so I can use the hardware EQ if I need to, I'm just pretty clueless when it comes to EQ, so I'm not really sure what I should be doing to limit my frequency range like this.

Can anyone give me an idea of what settings I need to change to make this work?

[Llorean]

I'll never yell at someone for putting something in General Discussion instead of the right place, simply because the rules say "If you're not sure, put it in General Discussion"

The upper and lower filters on the EQ are actually high shelf and low shelf filters, so in theory you could decrease the gain on those two to minimal, set them at the appropriate places, and all frequencies above #4, and below #0 (or are they 5 and 1? I don't use it much) would in theory be removed (or lowered dramatically. But I don't think that's likely to solve your problem.

What have you done that suggests it's not caused by loudness? What volume level is your iPod at?

[Stev]

Makes sense. I guess I was also a little cautious because it's only just a Rockbox question and more of a general question about EQ.

I'm at work now so I can't really test, but I will give that a try on the way home.

Because this accessory makes use of the line-out pin on the dock connector (or perhaps not 'because of', but regardless) the volume control on the ipod doesn't affect the volume sent to the FM broadcaster. I've also noticed (although this is _much_ harder to quantify) that the distortion doesn't appear to come in on peaks in loudness in the music - that is, it could be a fairly quiet section and I still get this distortion.

Two other (somewhat) related questions:

1. What's the Q setting and do I need to alter anything on that?
2. I'd prefer to use the hardware EQ if possible (I'm assuming this puts less strain on the processor & battery, although I could be mistaken) but the high and low shelf filters have only a small selection pre-defined frequencies. Is this a limitation of the actual hardware, or of the Rockbox implementation of the hardware? Just curious.

[Llorean]

Hm, it's true if the the FM transmitter is designed to use the line out from the dock then it should be fine at 0dB. Of course, then it should also be fine for the full frequency range of the iPod. You *might* try going into the EQ settings, and change the preamp to -10 or so, and see if that clears up your distortion problems right there (it's possible that the retail firmware doesn't actually put the line out at 0dB, but something slightly lower because of the innately volume-capped nature of the retail firmware).

1) The Q settings shouldn't really ever be altered on the high and low shelf features unless you know precisely what you're doing. In terms of the EQ, the Q is sorta how "wide" the range affected by the EQ is, with a lower Q meaning a wider range. It works differently for the shelves at the top and bottom, but I'm not really sure how (this is well outside my knowledge).

2) I've been told the choices for the hardware EQ are limited because of the hardware itself only offering those choices.

[soap]

I have to drive my line-out connected FM transmitter with -6dB of preamp to prevent clipping.

[Llorean]

Oh man, I hope we're not driving the line out at +6 instead of 0... Y'know, since +6 happens to be the max on iPod.

[soap]

I would not lose sleep over this crappy FM modulator.

[Llorean]

While I'm not so concerned about the FM Transmitter (so long as a workaround can be found to get it sounding right)  I would be concerned if our "line out" wasn't actually outputting at line level for some reason.

[Stev]

Hey! Whose FM modulator you callin crappy?!

Well, as far as I can tell, it's worked. I did do both the -6db precut and the high and low shelf filters at the same time though so I can't be sure which one worked. I also can't check and/or confirm because my ipod died partway through testing. I tried loading up a song I knew was problematic and it wouldn't play, just showed on codec. I was gonna try rebooting it, but once it shut down it wouldn't start up again. Won't even connect on USB

Anyhoo, I guess this needs its own thread, I just wanted to thank you guys for the advice and let you know that it seems to have worked.

[Llorean]

Just plug your iPod into your computer for about an hour, then flip the hold switch on then off again, then hold Menu+Select for about 30 seconds, and it should start up. I imagine your battery just died.

[saratoga]

Your FM radio probably can't produce any frequencies above 10-14KHz or so anyway because of the bandpass it uses, so changing the EQ in rockbox probably won't make a difference unless you have a very odd radio.

[soap]

While I'm not so concerned about the FM Transmitter (so long as a workaround can be found to get it sounding right)  I would be concerned if our "line out" wasn't actually outputting at line level for some reason.

I was simply trying to say that the behavior of my "crappy" FM transmitter should not be the cause of any loss of sleep (just because it wants -6dB preamp doesn't mean anything).  Its behavior is questionable in many regards.

[Stev]

Llorean, you are my hero. That is to say, you were absolutely right

If anyone's curious, I'm going to set up a couple of EQ presets to do some more testing to see whether it's the high and low shelf filter settings or the precut that seems to have fixed my problem.

Thanks again to all who have helped with both problems.

[Llorean]

*puts his money on the Precut*

[Stev]

We have a winner! Precut it is. Although I have to crank it up to about -12dB (or should that be crank it down?) to get rid of the distortion altogether.

Thanks again guys, glad to finally have this sorted. Although with 2 crappy FM transmitters now needing the precut it might be worth checking the level of the output on the lineout pin.