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SP-1a Speech Processor
- Thread starter mag69
- Start date
DO NOT confuse Lou Franklins DSP with the MUCH better SP-1A! CB City's processor is more or less a clipper circuit with user-defined filtering on the output. The idea is ok IF selective real-time filtering was practically possible, but that takes digital processing to accomplish, and is way out of the league of the average CB-er, or even ham operator for that matter. The SP-1A on the other hand is basically a glorified mic preamp and compressor/limiter, just like you find in any modern (last 30 years or so) CB radio from the factory. Its not EXACTLY like those built-in circuits, but it does the same exact job. It just does it MUCH, MUCH better.
mag69 said:
Why a SP-1A on a AM Radio? I have to ask.. On a SSB radio I can see a SP-1A .. AM ???
DTB Radio said:
Actually, the DSP IS a clipper. Clipping, although it gets a bad rap, is the single most effective tool one can use to increase loudness. If you don't believe me hook a scope up to the audio output of your AM/FM broadcast radio. Lou's DSP design could be better if it included an AGC/Compressor/Limiter and temperature compensated the diodes, (then your clip threshold will not change with the weather) but that would add to the complexity and the cost. I have not seen the schematics to the SP-1A but if it is what you say, perhaps someone should run an SP-1A followed by the DSP.
73
riq165
Clipping does increase loudness, but also adds tons of harmonic distortion, hence the need for Lou's user-selected output filter cap. Even with a large cap value, it isn't very effective, because it still allows harmonic distortion from the lower voice frequencies to get through. An active compressor/limiter, properly set, will do the same job with none of the harmonic distortion associated with clipping. I've played with the DSP as well as the SP-1A, and the audio quality difference is remarkable. The DSP makes a radio sound like its AMC is disabled.
The general idea is to make sure that all voice frequencies are amplified to maximum levels (but not over the point of distortion) no matter what levels they are at the input of the system. Both the clipper and compressor/limiter do that roughly equally. The difference is in the quality of the output signal compared to the input signal.
The general idea is to make sure that all voice frequencies are amplified to maximum levels (but not over the point of distortion) no matter what levels they are at the input of the system. Both the clipper and compressor/limiter do that roughly equally. The difference is in the quality of the output signal compared to the input signal.
The output filtering in any clipping system is the hard part. If you do too much filtering of the generated harmonics, you are undoing the clipping and the output will overshoot. Once it overshoots... well then you have failed to achieve your goal. Properly done, (the knobs are not at "Eleven") you can get away with quite a bit of harmonic distortion before it becomes truly offensive. The point I was trying to make in my earlier post is that audio clipping, in the context of CB radio audio, is not as bad as pulling all the limiters out of a radio cranking the power mic up until you can hear the wallpaper then letting the radio do the clipping via overload... oh ya lets drive a class C amp with that set up.
I would go with DTB's judgment on this one. Sounds like he has played with both of these products. If you get the sound you are after, that's all that matters in the end. I think too he would agree that you should have a bypass switch installed so you can turn it on or off.
73,
riq165
PS-Hey DTB, do you have a schematic of the SP-1? I am curious to see what it is doing.
I would go with DTB's judgment on this one. Sounds like he has played with both of these products. If you get the sound you are after, that's all that matters in the end. I think too he would agree that you should have a bypass switch installed so you can turn it on or off.
73,
riq165
PS-Hey DTB, do you have a schematic of the SP-1? I am curious to see what it is doing.
No, I don't, but if you pay really close attention to its action on an audio waveform, its more or less just a VERY glorified AMC (modulation limiter) circuit, like you find in a CB radio. It adds a nice, clean preamp, as well.
mag69 said:
Lou's dynamic processor works well-- but the thing is you really have to add alot of caps in the audio line everywhere to combating the squeeling and it take a bit to get rid of the squeels-- I use them just because I have alot of the IC's-- the spa-1 is the better one
I think the biggest draw back with Lou's compressor is that its prone to squeel and it takes a bit of work to get the squeels out-- its a real persnicky processor that can be made to work but with ALOT of playing around-- an even then at REALLY high compression levels its prone to start to distort from the back to back diode arrangement he uses (same type of circut used in distortion pedals for guitars)
It still has alot of gain and compression and I've sort of got thye hang of making them work well and I've put them in lots of radios
But for ease of installation, and overal performance the spa-1 is the better of the 2--- the spa-1 is not as picky and prone to sqeals as Lou's dynamic
Lou's can be made to work well-- but the spa-1 is easier to get working and hooked up and doesn't like to squeal as easily which is a big problem with the dynamic-- getting rid of the squeals
I build, use, and instal dynamic processor regularly just because they are cheap and easy to build and they do work decently and are very loud-- the struggle is when you first install one and have to supress alot of squealing -- it can be made to work really good and without squreals but the spa-1 is so much easier
So I'm going to say the spa-1 is the better of the 2 just because of the squealing prooblem that has to be surpressed with the dynamic
ZR2 said:
I definatly think so-- I agree with DTB with the audio quality-- once the squeal problem is overcome there is still not very good audio quality from the design-- back to back diodes--
And again DTB hit the nail on the head with the harmonics--
SPA-1 for sure--- actually I ran into an old HyGain compressor design that worked really well and clean-- certainly lots of other compressor/processor available besides those 2, and slo schematics available for different designs if you homebrew your own gear
Get the SPA-1 or check into the amateur market for others
the heart of the SP-1A is a quad op-amp. only three of the four op-amps are used. the first one is the preamp, the second one is used as a quasi-logarithmic limiter and the third op amp section is configured as an audio bandpass filter from roughly 300 - 3,000 hz.
if i remember what device is used i'll append this post and add the pertinent information. the device can also be used in the receiver section to provide expander capabilities. they used to market a dual use (tx and rcve) "compander" called the VSB-1. the expander capabilities it provided for the receiver section were incredible.
if i remember what device is used i'll append this post and add the pertinent information. the device can also be used in the receiver section to provide expander capabilities. they used to market a dual use (tx and rcve) "compander" called the VSB-1. the expander capabilities it provided for the receiver section were incredible.