TubeNet

I did some research on the business of FM transmitters for spreading internet radio content around the house. It seems that most use the same chipset, but some have better implmentations than others.

One that's different operates as a plug-in USB set of speakers and so requires drivers and onscreen menus to operate. I was a bit leery of this one because I wondered about the sound quality and compatibility issues (I'm running the transmitter off of a Linux server).

I settled on the little Linex model, which takes 3 AA batteries and allows you to select from one of seven FM frequencies (all at the high end of the FM spectrum) via a set of DIP switches. Plug the unit into your computer's audio line-out jack adjust the volume and you're all set to go. Less than $18 if you look hard and has a 150 foot range. Forget about the Tunecast and "cigarette lighter" transmitters; most have only a 10 foot range on a good day.

It worked pretty well, with two drawbacks. One is the life of the 3 AA cells--the specs say 100 hours, but I got considerably less than that. Linex also sells a USB-powered model, which seems to do nothing more than borrow its power from the USB plug (still requires an audio input). Since I've got a well-stocked "hellbox", I grabbled a USB cable and added it to the unit in place of the batteries, figuring that the 5 volts from USB is close enough to 4.5 volts produced by the batteries. It works fine, so problem one out of the way.

The second was that there was a fair amount of hiss present from the Soundblaster 32-bit audio output. You can hear it suddenly "click on" when the drivers for the SB load. I figured that I could minimize the hiss some if I could run the SB at its highest "distortionless" level and attenuate the output externally, between the PC and the transmitter. A simple 1K pot does the trick and the hiss level is much reduced, although still audible in very quiet passages.

Well, maybe it's not an "off the shelf" solution, but I think it's the best currently-available option for me.

Chuck(G) wrote:It worked pretty well, with two drawbacks. One is the life of the 3 AA cells--the specs say 100 hours, but I got considerably less than that.

Since you have the USB power solution, it not applicable, but NiMH rechargables last much longer, look for 2000mA/Hr or better. Anyone without the "skills" to hook up the USB power solution take note

The second was that there was a fair amount of hiss present from the Soundblaster 32-bit audio output. You can hear it suddenly "click on" when the drivers for the SB load. I figured that I could minimize the hiss some if I could run the SB at its highest "distortionless" level and attenuate the output externally, between the PC and the transmitter. A simple 1K pot does the trick and the hiss level is much reduced, although still audible in very quiet passages.

That's a known problem with most consumer level sound cards. S/N sucks. It's even worse on the A/D side. They are getting better though. Do you have the specs on the card you have? A lot of low end cards (and on the MB chips) don't even list them.

The SB Live 24 claims 100dB. But I don't see specs on the SB Live 5.1
Their high end Audigy 4 Pro doesn't say in it specs page. Go figure.

Right now, for my test setup, I'm using a SB CT4780 cheapie for the audio. There are some pretty good deals on EMU 0404's out there and it may be "good enough". After all, I'm dealing with off-the-net audio and plain old FM quality once it gets into the transmitter.

I've tried RightMark Audio Analyzer on the SB card and it says about 100 db S/N ratio, but that's nowhere near right. I wonder how they get their numbers...

Chuck(G) wrote:I've tried RightMark Audio Analyzer on the SB card and it says about 100 db S/N ratio, but that's nowhere near right. I wonder how they get their numbers...

I'll bet they're measuring the line level, not the amplified signal. I've not done any tests to prove it to myself, but I've always figured those teensy amps and lowest-bidder potentiometers can't be that well shielded, and must add a fair amount of noise.
___________________________
Joe Baker, who doubts whether the best of these cards can deliver 100 db SNR after the amp & the (cheapest available) pot.

Joe Baker wrote:

Chuck(G) wrote:I've tried RightMark Audio Analyzer on the SB card and it says about 100 db S/N ratio, but that's nowhere near right. I wonder how they get their numbers...

I'll bet they're measuring the line level, not the amplified signal. I've not done any tests to prove it to myself, but I've always figured those teensy amps and lowest-bidder potentiometers can't be that well shielded, and must add a fair amount of noise.
___________________________
Joe Baker, who doubts whether the best of these cards can deliver 100 db SNR after the amp & the (cheapest available) pot.

The line out tends to suck too. That's where Chuck noticed the problem first. You righht though the amps are even worse. I love the THD number thay give. That must be with minimal gain.

Another thing Chuck, When the drivers load they reset a lot of stuff on the card. I've alwas noticed pops on my cards. Some have decent power-on defaults, but many don't. How much noise do you get at the BIOS config screen?

Best bet is have the amp off untill the drivers load. Then have the audio muted untill there's a signal. Once you have a signal, S/N is OK, at least for FM broadcast, but no signal tends to have a lot of noise. If it an actual card, you can try moving it to another slot. EMI can wreak havoc on the poorly/non sheilded components, which is most of them

Maybe make a sheild? Check the power into the card (probe the PCI/ISA bus)?

Perhaps you only real solution for noise would be an external D/A. Maybe a USB audio "card", with external power? Looks liek Creatrive has several that are USB 1.1, and bus powered. Not sure of status of Linux dirver for any of them though.

I know several "Professional" cards were really external. There was a PCI/ISA connetor in the box, that forwarded the signales to the external box, and it had a seperate power supply to get away from all the noise issues inside a computer case.

ThomasDodd wrote:Another thing Chuck, When the drivers load they reset a lot of stuff on the card. I've alwas noticed pops on my cards. Some have decent power-on defaults, but many don't. How much noise do you get at the BIOS config screen?.

Well, the output's silent until the driver loads, even in the BIOS config. It seems silly to use the on-card amp if I don't have to--just adds more noise. Maybe I'll probe around and see if I can get a decent signal at the input to the amp.

Well, guess what?

After determining that I wasn't going to get to the bottom of the noise issue easily, I decided to throw in the towel and install the transmitter on the server (a rather geriatric, but built-like-a-tank Compaq Deskpro EN) and take whatever I got. This thing uses a lowly onboard ESS ES1869 chipset for sound. I'm running Red Hat 9.something.

The noise is hardly audible at all--at least no worse than standard FM reception.

Works like a charm, so far. Right now there's some very weird stuff on BBC Radio 3 playing...


Go figure...a cheaper soundcard works better.

Chuck(G) wrote:Well, guess what?
The noise is hardly audible at all--at least no worse than standard FM reception.
Go figure...a cheaper soundcard works better.

Not overly surprising. Creative has the name, so many will buy based on the name. No comparisons done, just the assumption that Creative is best, and no need to actually find out.

Different chip? I know the ESS chips were pretty good. Not sure what the SB might have, emu10k1? I've got a no-name card with an ES1370 that works well. I've never actually owned a Creative card.