Re: [TenTec] AM Receiver distortion in Orion II

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

hi jerry,

I hope you don't mind if I violate your copyright by reposting some of your 
stuff.

> Hi,
>
> I'll try to explain this. think of a dsb circuit as a data processing 
> circuit.
>
> If you have two circuits, one using 16 bit processing and the other 24 bit, 
> but they both have the same clock speed, then the 24 bit circuit processes 
> more data for a given amount of time.

<<<NOT AT ALL! The words are wider which makes the dynamic range is greater
but the 24 bit processor works one instruction per clock cycle just like
the 16 bit processor. Sometimes the wide processor word makes a wider
instruction word (but its a characteristic of DSP that the instruction
bus/memory and the data bus/memory are independent) that can do more
complex operations per clock cycle but that's not a given going from 16
to 24 bit DATA.>>>

Correct.  I apologize for my misleading statement.  After I sent it I 
realized it was not correct but unfortunately I don't have the free time 
needed to continually interact with the reflector.
DSP systems with 24 bit words are vastly more precise in their ability to 
represent analog waveforms by virtue of their brute force:  a 16 bit word as 
2**16 = 65,536 possible permutations; a 24 bit word has 2**24 or 16,777,216, 
way more than two orders of magnitude
greater values available for assignment in an algorithm.  this means less 
approximations and a better reproduction when the signal is converted back 
to analog for listening.

With a greater word length in the Orion, I would hope Ten Tec would take 
advantage of it.  With A to D conversion of an audio signal, it is true that 
there is an upper limit to the sampling rate as far as fidelity is 
concerned.  Let's say for clarity, you have a 4 bit dsp system.   You 
therefore have 16 steps available to assign to an analog waveform from its 
highest to lowest voltage.  there's an upper limit to your sampling 
frequency beyond which you can sample at x-ray frequencies but you are still 
going to have to round off to one of the 16 steps, therefore you have no 
improvement in fidelity unless your word length increases and nothing is 
gained by increasing the sampling rate unless your analog signal frequency 
dramatically increases.

You responded to my comment on sampling rate and word length with the CD 
player with a dismissive "NOPE."  I will merely respond that in theory at 
least, you are incorrect.  If the sampling rate for a 24 bit system were 
slow enough, far below the upper limit rate of the 16 bit system, the 24 bit 
system's reproduction would suffer, however it would have to be extremely 
slow I admit.

> That means the digital simulation of an analog waveform for a fixed time 
> period has greater precision because it is sampling at the same rate, but 
> processing it more rapidly, 1/3 more for each clock tick.    Processing 
> bigger chunks of data at a time means you can do things like employ more 
> complex processing algorithms that can result in a more faithful analog 
> result.

<<<Not so.>>>
I disagree.

> Assuming a CD player is a 16 bit system, it could be extremely high 
> fidelity, if it's clock speed is extremely high because then, it would be 
> sampling and processing more data than a 24 bit circuit on a much slower 
> speed for a given amount of time.

<<<NOPE.>>>

> So besides the word length, we also need to know the clock speed for the 
> dsp circuits, which I for one, do not know off hand (I think the 870 dsp 
> runs at 10 MHz but don't hold me to that).
>
> a few folks have mentioned that  the Orion is (depending on who you talk 
> to) a 24 or 32 bit dsp processor.   That's great and I humbly apologize for 
> implying anything else, but its clock speed is also important and in any 
> event, there seem to be other components that limit its flat frequency 
> response.

<<<Like the A/D and D/A word widths.>>>
>

--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer

73

rob / k5uj

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