On Wed, 30 Jan 2013 18:45:50 -0500, rickman wrote:
> On 1/30/2013 12:07 PM, Tim Wescott wrote:
>> On Tue, 29 Jan 2013 18:02:06 -0500, rickman wrote:
>>
>>> On 1/29/2013 12:34 PM, Tim Wescott wrote:
>>>> So I should stop using "radians", "radians/second", etc., in any
>>>> calculations involving angles or frequencies?
>>>>
>>>>
>>> Actually, it's not a "pure" number.
>>
>> Yes and no. In digital-land, it's a pure number, regardless of how it
>> arrived.
>
> I can't say I get that. Are *all* digital numbers unitless? If not,
> then why this one?
How can a digital number be anything other than unitless? Digital
numbers are, by definition, numbers. Numbers are, by definition,
unitless. Unless you know way more about mathematics than anyone else on
earth.
>>> Each count of the ADC represents some voltage and that is always
>>> considered when doing signal processing,
>>
>> Now how can you say that? How many signal processing projects have you
>> been involved in? How many products that include signal processing of
>> your authorship have successfully shipped?
>>
>> I've been at this game for well over 20 years. Yes, the ADC input
>> (which isn't always a voltage, especially if you consider things like
>> encoders and resolver-to-digital converters to be "ADC"s). But there
>> are significant stages in the design of a signal-processing chain where
>> you want to take each signal in its native units, and the native units
>> of a number isn't volts.
>
> You keep saying this, but you really aren't explaining. BTW, I think
> your playing the "experience" card is a bit pompous. Can no one with
> less experience than you have a valid opinion? BTW, you really don't
> know how much experience I have. It's not like I woke up this morning
> and decided to consider ADCs.
I know that you've asked some pretty damned naive questions regarding
your WWVB receiver, and that you stated that you were a hobbyist, and
that you have a history on this group and others of refusing to accept
good advice.
And, of course, getting bent out of shape when this is pointed out.
I am explaining. My transmitter is working. You aren't listening. Fix
your receiver.
> If the user is measuring a signal from an antenna or an amplifier, would
> volts not be the unit the input is scaled to? In the part quoted below,
> I said, "unless that voltage represents some other quantity like...
> power." So there may be other units for the input. But to say it is
> always unitless by definition is so far unsupported.
OK. You're clearly smarter than me. So, oh experienced, smart one,
educate me:
What are the units of 10?
What are the units of 100?
What are the units of 42?
Whatever your answer is -- that's the units of the number that comes out
of an ADC.
>>> for example calculating dB between two values of ADC readings, the
>>> multiplier will be 20, not 10 because it the ADC is measuring
>>> voltage...
>>> unless that voltage represents some other quantity like... power.
>>>
>>> The measurement has some units. The question is about what to label
>>> the ADC reading before it has been scaled, that's all. I say label it
>>> by the quantity the input to the ADC represents.
>>
>> If you have a block diagram of a large system, and you are analyzing
>> the signal flow, it is an extremely good idea to get very picky about
>> dimensions, both of your signals and of your gains.
>>
>> So, for instance, an ADC will have an input that is a voltage, an
>> output that is a dimensionless number, and a gain whose dimensions are
>> volts^-1. Similarly, a DAC might have an output that is a voltage, an
>> input that is a dimensionless number, and a gain whose dimensions are
>> volts.
>
> I can't say I follow that. What is wrong with having units of volts on
> the digital signal and the converters having unitless gain? If you want
> to make a point, please give some justification for it other than, "this
> is the way I found it to be best".
Because the digital signal is a _number_. Which is inherently _unitless_.
What's wrong with having units of volts on a tape measure? Because it's
totally WRONG, that's what's wrong with it!
>> But expressing gains in volts^-1 or volts can be confusing: hence, I
>> prefer to speak of an ADC gain as counts/volt (or fullscale/volt -- I
>> use one or the other depending on what I'm trying to convey and how
>> many short cuts I feel I can take), and a DAC gain as volts/count.
>> That clarifies things immensely, much as one may see an amplifier's
>> gain labeled as "volts/volt", even though technically the amplifier's
>> gain is dimensionless.
>
> I think the issue the OP is discussing is how to make the code more
> clear. In that case I see no reason not to give a label to the digital
> values that represent the quantity being measured.
"Before calibration I tend to refer to it as..."
"Before calibration"
Let's try that again: "Before calibration"
What is the reading, in volts, of an uncalibrated meter that reads "10"?
Remember: you're way smarter than me, so you _do_ have an answer.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com