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bytheway
2004-Aug-01, 07:28 AM
This was a random thought I had the other day while driving in the car and listening to the radio. I don't know too much about the Doppler Effect nor too much about AM and FM radio...but then I remembered the guys at the Bad Astronomy Bulletin Board and decided to try them as a sounding board. And...my login still works!

Anyway, my thought was this...can the speed of the car effect the sound pitch on the radio?

As I was driving and listening, I heard some interesting interference that seemed to be related to my RPM (higher RPM, higher pitched interference). Obviously, this has nothing to do with the Dopler Effect -- it's just what got my brain juices flowing. (Someday I'll look into what was causing that interference on that particular day, an event I cannot seem to reproduce...)

But, back to the subject. Of course, a car drives relatively slow compared to radio waves, and really, that would only affect the reception frequency, not the pitch. At least, that's my understanding on AM radio -- amplitude modulation.

But what about FM -- frequency modulation? I'm not sure how that works, but by the name, I would suspect that the slight changes in frequency change the pitch the speakers produce...so wouldn't the speed of the car affect that pitch? At least, I've never heard a difference between the 0-75 MPH (I try to drive the speed limit) range.

Then again...I don't have perfect pitch...

What are your thoughts?

--BTW

(p.s. I remember doing redshift calculations in my Modern Physics class ... about 3 years ago. Yeah, right ... like I remember how to do that any more...)

darkhunter
2004-Aug-01, 09:29 AM
As I was driving and listening, I heard some interesting interference that seemed to be related to my RPM (higher RPM, higher pitched interference). Obviously, this has nothing to do with the Dopler Effect -- it's just what got my brain juices flowing. (Someday I'll look into what was causing that interference on that particular day, an event I cannot seem to reproduce...)

Sounds like interference from you ignition system....I had a truck that was like that once until I got around to replacing the spark plug wires and ignition lead....

Donnie B.
2004-Aug-01, 12:23 PM
This was a random thought I had the other day while driving in the car and listening to the radio. I don't know too much about the Doppler Effect nor too much about AM and FM radio...but then I remembered the guys at the Bad Astronomy Bulletin Board and decided to try them as a sounding board. And...my login still works!
Welcome back!


Anyway, my thought was this...can the speed of the car effect the sound pitch on the radio?
In a word, no. FM encodes sound by varying the frequency of a carrier wave around some average value. The motion of a car could, at most, shift the average frequency slightly, which would "de-tune" the station slightly.

However, the amount of frequency shift that a car's speed can produce is quite tiny compared to the carrier frequency of the station. If your car were going fast enough to notice any Doppler shift in an FM signal, you'd have a lot more to worry about than losing your reception!

ngc3314
2004-Aug-01, 06:39 PM
This was a random thought I had the other day while driving in the car and listening to the radio. I don't know too much about the Doppler Effect nor too much about AM and FM radio...but then I remembered the guys at the Bad Astronomy Bulletin Board and decided to try them as a sounding board. And...my login still works!
Welcome back!


Anyway, my thought was this...can the speed of the car effect the sound pitch on the radio?
In a word, no. FM encodes sound by varying the frequency of a carrier wave around some average value. The motion of a car could, at most, shift the average frequency slightly, which would "de-tune" the station slightly.

However, the amount of frequency shift that a car's speed can produce is quite tiny compared to the carrier frequency of the station. If your car were going fast enough to notice any Doppler shift in an FM signal, you'd have a lot more to worry about than losing your reception!

Typical FM carrier frequences are around 100 MHz, and broadcast stations are spaced (at least in the US) no less than 0.2 Mhz apart. If we take a plane just below Mach 1 at 1000 km/hr for ease of calculation, it will see a maximum Doppler shift of 100 Mhz *(v/c) = 9 Hz, which is within the range that tuners automatically track variations simply due to such factors as changing temperatures of the electronics (since internal frequency standards nay change with such environmental effects).

For satellite velocities, Doppler shifts do become an issue for use of the GPS system in navigation - there are builtin limitations on some civilian (commercial) receivers, among other things to make it more difficult for any party not in the club already to build accurate delivery systems for WMDs.

bytheway
2004-Aug-01, 07:45 PM
Thanks!

I didn't think the carrier wave would be affected too much (we ARE talking about v/c, after all).

However, my thought was whether or not it might play around with the variations on that wave that cause the sound.

Well, obviously, I've never heard any variation, so it doesn't affect the sound much, if at all.

So, then...do the variations go unchanged/virtually unchanged by the doppler affect? Or is it that the changes move along side the carrier wave, thus preserving the sound?

And the other thought...a car isn't moving parallel with the radio waves, I'm guessing...so even if it were moving at MACH 5, no effect would be seen...

Just my thoughts.

--BTW

George
2004-Aug-02, 12:21 AM
So, then...do the variations go unchanged/virtually unchanged by the doppler affect? Or is it that the changes move along side the carrier wave, thus preserving the sound?
My guess is the carrier wave is removed so only the modulation is used. I would also guess it would be off a tiny bit due to Doppler. If 1000mph is 9 hz, 100 mph would be .9 hz. Who would notice?



And the other thought...a car isn't moving parallel with the radio waves, I'm guessing...so even if it were moving at MACH 5, no effect would be seen...
Apparently, JPL saw a Doppler problem in communicating with the probe so they planned the launch at a trajectory where Cassini will be traeling more perpendicular to the probe for the reason you just gave. Of course, you would have to fly in a circle around the station to minimize doppler.

bytheway
2004-Aug-02, 12:35 AM
If 1000mph is 9 hz, 100 mph would be .9 hz. Who would notice?


That reminds me of a true story of a college music professor (I can't remember his name, but this story was told in his presence and verified by himself, so it's true).

The basic tuning note for a symphonic orchestra is A440 (440 Hz that plays the A above middle C). The standard may differ between country and orchestra (A436-A444 or so), but A440 is the most common.

Well, this professor opened up the door to the room, only to find someone tuning a piano (a very hard thing to do -- I've tried before). To make it easier, they like to have the room completely quiet, the door shut, and no distractions. So, the professor begged pardon, whereupon the person tuning asked, "I hear you have perfect pitch. Mind if we put you to the test?"

The man was tuning using a digital meter that measured the frequency of a note with a precision of 0.1 Hz. He gave the tuning wrench to the professor and told him to tune the A key.

Which he did...sitting there perfecting the pitch for several minutes.

Finally, he gave up: "I got it as close as possible, but it still sounds a little sharp [high-pitched]".

The tuning man played the note and looked at his electronic tuner, which registered the note at 440.1 Hz.

Most probably won't notice, but I think HE would...

--BTW

ToSeek
2004-Aug-02, 02:50 AM
I went to college with a guy who was so sensitive to pitch that he couldn't stand to listen to piano music because of the slight tweak done to the frequencies to keep all the notes at the same relative ratio.

ngc3314
2004-Aug-02, 05:44 AM
I went to college with a guy who was so sensitive to pitch that he couldn't stand to listen to piano music because of the slight tweak done to the frequencies to keep all the notes at the same relative ratio.

So that would mean he could stand to listen to either electronic music, string quartets, or trombone ensemble music?? I have seen folks with (near as I could tell) perfect pitch look like their spines were about to curl hearing things that sound pretty good to me.

genebujold
2004-Aug-02, 02:39 PM
This was a random thought I had the other day while driving in the car and listening to the radio. I don't know too much about the Doppler Effect nor too much about AM and FM radio...but then I remembered the guys at the Bad Astronomy Bulletin Board and decided to try them as a sounding board. And...my login still works!

Anyway, my thought was this...can the speed of the car effect the sound pitch on the radio?

As I was driving and listening, I heard some interesting interference that seemed to be related to my RPM (higher RPM, higher pitched interference). Obviously, this has nothing to do with the Dopler Effect -- it's just what got my brain juices flowing. (Someday I'll look into what was causing that interference on that particular day, an event I cannot seem to reproduce...)

But, back to the subject. Of course, a car drives relatively slow compared to radio waves, and really, that would only affect the reception frequency, not the pitch. At least, that's my understanding on AM radio -- amplitude modulation.

But what about FM -- frequency modulation? I'm not sure how that works, but by the name, I would suspect that the slight changes in frequency change the pitch the speakers produce...so wouldn't the speed of the car affect that pitch? At least, I've never heard a difference between the 0-75 MPH (I try to drive the speed limit) range.

Then again...I don't have perfect pitch...

What are your thoughts?

--BTW

(p.s. I remember doing redshift calculations in my Modern Physics class ... about 3 years ago. Yeah, right ... like I remember how to do that any more...)

Absolutely!

Just nothing you would notice.

Excluding time dilation effects (minimal at 70mph), we find the frequency of a 1kHz tone would, if you were driving at 70mph straight towards the station, to increase to...

You're doing 0.019444444 miles per second. The speed of the radio wave is travelling 186,000 miles per second, with a doppler differential of 1.000000105, or .0000105%.

So, your 1kHz tone would become a 1.000000105kHz tone.

To see if YOU can discern the difference in such a small tone, go here: http://www.ucihs.uci.edu/hesp/newversion/webtest/frequency/ie_index.html

My best score was a 5.6, which means I can, on average, discern with a 50% accuracy a pitch difference of 1000Hz and 1005.6Hz.

This means that I've only got to improve my pitch discrimination by a factor of 53,568 times before I'll be able to hear the EM doppler shift of a 1kHz tone wile travelling 70mph...

bytheway
2004-Aug-02, 03:39 PM
What a cool little test!

Apparently my musical training has paid off...I pulled out slightly better at a 4.3Hz score.

Thanks for the info!

--BTW

tlbs101
2004-Aug-02, 04:45 PM
This thought was the subject of an April Fools article written about 20 years ago (and maybe before that).

A person wrote that as he was travelling away from an FM radio broadcast tower, that he would have to re-tune his radio (on that station) lower, but as he moved toward the tower he would have to re-tune the radio higher. While technically this is correct, the amount of correction needed is miniscule compared with the noise errors inherent in the radio.

genebujold
2004-Aug-02, 05:06 PM
This thought was the subject of an April Fools article written about 20 years ago (and maybe before that).

A person wrote that as he was travelling away from an FM radio broadcast tower, that he would have to re-tune his radio (on that station) lower, but as he moved toward the tower he would have to re-tune the radio higher. While technically this is correct, the amount of correction needed is miniscule compared with the noise errors inherent in the radio.

Not only that, FM stations operate around 100MHz, but channel separations are complex, varying with both power and distance between stations: http://www.geocities.com/Area51/Nebula/3736/73207.html

Nevertheless, the minimum separation between channels is 200kHz.

Give the math, above, and again travelling at 70mph, your tuning would drift a "whopping" 0.0105kHz, which is just 1/19,047 of the minimum channel separation.

So, if you can dial in 20,000 discrete steps between two adjacent channels on that FM dial, then you might be in need of an adjustment. Otherwise, forget it - you're probably covered.

George
2004-Aug-02, 05:45 PM
...Give the math, above, and again travelling at 70mph, your tuning would drift a "whopping" 0.0105kHz, which is just 1/19,047 of the minimum channel separation.

So, if you can dial in 20,000 discrete steps between two adjacent channels on that FM dial, then you might be in need of an adjustment. Otherwise, forget it - you're probably covered.
Just to carry it a tad further and to eschew obfuscation....the 105 Hz shift represents a 0.005% frequency shift, therefore, the modulation would shift by the same 0.005% yielding a 1/2 hz shift and not 105 Hz. [This is implied in genebujold's statement, right?]

Considering how hot my radio gets, I wish Doppler shift were the only problem. :)

worzel
2004-Aug-02, 06:09 PM
I went to college with a guy who was so sensitive to pitch that he couldn't stand to listen to piano music because of the slight tweak done to the frequencies to keep all the notes at the same relative ratio.

So that would mean he could stand to listen to either electronic music, string quartets, or trombone ensemble music??
I think brass instruments are not actually equally tempered (unlike pianos, pretty much anything electric, fretted, or unfretted strings if conventionally tuned) because they use the physics of natural harmonics to get all the different notes. That was always my excuse in the school orchestra for being out of tune anyways :wink:

Charlie in Dayton
2004-Aug-02, 11:41 PM
And of course, let us not forget the 19 kHz pilot tone that FM stereo stations use to keep your receiver locked on the station. The tone gets filtered out so you never hear it. Unless your vehicle speed is an appreciable percentage of the speed of light, you'll never hear any difference -- your radio sorta kinda auto-tunes itself to where it needs to be to stay on frequently.

I remember that old Monkey Ward Airline hi-fi set (the one with the turntable and four-pound tone arm) in the basement...no FM stereo, just FM...every half hour or so while Mom was doing the laundry, she'd have one of us go over and tune the classical station back in...it had drifted off again...this was back in the days before AFC on FM (who else remembers that?).

bytheway
2004-Aug-03, 02:34 AM
Is there any record of Apollo flights having to deal with the doppler effect on their communications? After all they are moving parallel with the signal and much faster?

To compensate the reception frequency would be easy...just broadcast on one and receive on the other. But if they used FM, the modulation would be affected.

Do we know if they used FM or AM?

-BTW