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EQ Advice

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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.

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