audio bandwidth limiter

[K6JEK]

How are you guys limiting your audio bandwidth? 

I haven't been.  Get a load of this picture from PR40, 528e, class H/class E.  I'm looking a little wide. I suppose I could just turn the knobs on the 528E.

[flintstone mop]

I am an owner of a 528E and 'turning the knobs' can really make ugly audio out of that device. I would bypass the EQ and daisy chain a quality EQ box with balanced in/outs and just lop off anything above 7kc........And somewhere in this forum there are birck wall designs for limiting BW.
Looking at my 528E I would guess that setting the HIGH EQ for 6.8khz and the octave BW at MAX and turn the output to 0dB would reduce the high freqs.
But what I see in your screen shot looks like you might do it from the software........Is this an SDR type radio??? Class H/E is an outboard TX you built or part of an SDR box?
Fred

[WA3VJB]

Multiband EQ after the processor, and very little compression of the highs.



Just roll it all off to stay friends with the neighbors.

Open it up when there's room.  The snapshot you've provided does not show anyone nearby, so there's room for the data rate you wish to transmit.

If you're controlling baseline with few if any overshoots, and the transmitter IMD is clean, then you're playing nice with others.

[Jeff W9GY]

I use a 1/3 octave analog audio eq.  Seems to do the job without much added noise or RF susceptibility issues.

[KA2DZT]

My HB xmtr has a 5KC low pass filter built into it.  No audio above 6KC.

Fred

[The Slab Bacon]

My HB xmtr has a 5KC low pass filter built into it.  No audio above 6KC.

Fred

What doesn't your 51 watter have built into it? ? ? Does it have a fridge and a beer cooler, it's big enough.....................................    

[K6JEK]

It's not an SDR. It's a QIX.  That spectrum shot was from another ham a hundred and fifty miles away.

Yes, there was plenty of room last night so letting the highs rip was fine with me and garnered terrific audio reports from the various jokers I was talking with.  I always get spectacular audio reports with this baby set up this way.  

Maybe I'll build a little outboard box of some sort for when I need it so I can leave all those knobs alone and flip some switch when I need to.  Turning the EQ off completely unfortunately, leaves just my muddy voice.

PS: Nice picture of the DAP 610 above.  Is it pretty or what?

[KA2DZT]

My HB xmtr has a 5KC low pass filter built into it.  No audio above 6KC.

Fred

What doesn't your 51 watter have built into it? ? ? Does it have a fridge and a beer cooler, it's big enough.....................................    

I thought about a fridge, but space was limited.

[K5UJ]

there's at least one article on how to make a LP filter for your audio in ER but I don't remember which issue.  the right way to do it is to use some kind of passive LP filter, maybe more than one with different cutoffs you can switch in for varying condx.  The quick and dirty way (what I do as I am lazy) is to get a couple graphic eqs, cascade them and sink all the sliders to max. attenuation at and above some cutoff frequency of your choosing, 4 or 5 khz.

that spectrum plot makes your sig so wide it is hard to see where yours ends and the others begin.  

It's a good idea to get an audio sig. gen and put your rig on a DL and look at the carrier with a scope.  Feed it audio sine wave and adjust level until cxr almost pinches off then sweep it up and down in frquency and watch mod. level to get an idea of the freq. response of the rig, then put your filter in the air chain and repeat to see how it does.  

Hams running broadcast peak limiters designed in the 1980s and later run into something called "pre-emphasis."  This is a design element introduced by the National Radio Standards Committee that essentially arranged for a boost of high frequencies in the audio up to a bit over 9KHz to compensate for consumer broadcast receiver rolloff in their passbands.  (some modern AM bc receivers start to roll off at around 3 KHz)  If a ham runs something like an Inovonics 222 or CRL peak limiter wide open it will boost his high frequency sibilance sounds.  We don't really need that for ham voice so I think it is a good idea to roll off the audio ahead of these boxes if they are being used.

You can really hear the NRSC pre-emphasis if you listen to a modern day broadcast station with an old wider passband receiver.  the consonant essing is so pronounced it is almost irritating. 

[steve_qix]

I use a 1/3 octave eq and reduce the highest few bands.  This seems to work quite effectively.  Also, you *could* build the LP filter from the PWM transmitters and put this ahead of the modulator.  That one is pretty sharp - 6 poles.

[K6JEK]

You know what I just noticed getting dusty on a shelf?  A DSP 599.  If I recall correctly, when off it's really off as in relay bypass and when on you can set arbitrary audio bandpass.  I think it even has line level ins and outs.

This is a sledge hammer to kill a fly but what the heck.  I have the sledge hammer.

[Opcom]

bah. all these active devices.. Been collecting various passive audio bandpass filters for a while. Little steel cans. Best one so far has been 200-4000Hz, but combined with others having responses here and there in the whole spectrum, it should either be workable, or maybe an experiment gone wrong. One of the two. The idea is to use a switch or set of switches to pick a bandpass, sort of like the convenience of the DSP stuff but without the digital aspects. Its not like a BW filter has to be perfect & there's lots of room for fun.

[K6JEK]

bah. all these active devices.. Been collecting various passive audio bandpass filters for a while. Little steel cans. Best one so far has been 200-4000Hz, but combined with others having responses here and there in the whole spectrum, it should either be workable, or maybe an experiment gone wrong. One of the two. The idea is to use a switch or set of switches to pick a bandpass, sort of like the convenience of the DSP stuff but without the digital aspects. Its not like a BW filter has to be perfect & there's lots of room for fun.

Little steel cans?

[W1DAN]

Hi:

Many of the above ideas do not limit your bandwidth deeply enough. I have a brick-wall filter that is both easy and effective.

After seeing that Stu AB2EZ modified a SCAF receive audio filter unit for use as transmission filtering, I built an 8 pole elliptical low pass filter using a Maxim MAX294 switched capacitor filter IC for my AM rig and set the cut-off frequency to about 6kc. Easily tuneable and works like a charm. I feed it processed audio so the relatively limited S/N is not an issue.

http://www.maxim-ic.com/datasheet/index.mvp/id/1443

Highly recommended and easy.

73,
Dan

[Opcom]

bah. all these active devices.. Been collecting various passive audio bandpass filters for a while. Little steel cans. Best one so far has been 200-4000Hz, but combined with others having responses here and there in the whole spectrum, it should either be workable, or maybe an experiment gone wrong. One of the two. The idea is to use a switch or set of switches to pick a bandpass, sort of like the convenience of the DSP stuff but without the digital aspects. Its not like a BW filter has to be perfect & there's lots of room for fun.

Little steel cans?

Yeah the little steel cans.. a few potted little bricks, etc. I know it is not a sharp cutoff but I think it's OK for my use. Two in series sharpen that up a lot. They are cheap when found in a box of junk or transformers at fests as long as the seller isn't greedy. Have to admit the ICs are very good, and tunable is the ticket.