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 Post subject: Frequency response of 30's radio
PostPosted: Sep Sun 18, 2011 6:06 pm 
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Has anyone ever measured the frequency response of 1930-1950 tube radios? What was the limiting factor, bandwidth or speakers? During the time Stereo Review actually measured anything about Am in receivers it ran the gamet from 20hz to 6000hz and as bad as 300hz to 1900 hz.


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Sun 18, 2011 7:21 pm 
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The figure of 20 Hz to 6,000 Hz is about as good as it's going to get; either in the 1930's or today, with broadcast AM.

Remember, an AM broadcast channel is 10 kHz wide, which means with two sidebands you can only do up to 5,000 Hz. Of course every time I bring this up, at least ten others will chime in and tell me that you can get more. Sure, but only because there's a rolloff -- so many dB down at 7,500, just about gone entirely at 10,000, so you can INDEED get 7,500 or 8,000 or whatever through an AM receiver, nyah nyah. Okay fine -- now let's duke it out between the hi-fi guys and the AM-is-great guys. ANYONE who reads freq specs on a hi-fi component knows that this response is PLUS OR MINUS 3 dB's. In other words if it drops it don't count, even though it's still there. An AM signal rolls off at a point of something less than 5,000 Hz. Exactly where that minus 3 dB mark is at, I'm not sure, but it's sitting somewhere in the range of 5,000-6,000 Hz. But that's the BROADCAST. The receiver is going to roll this off even more, and I'd venture a guess that when all is said and done, with most radios you're looking at a top end of 4,000-4,500 Hz. Car radios are probably less, and some better home radios a little more. A few models have a variable Q which can give you the full range as long as it doesn't cause bleed from other stations, but again you're limited to what the transmitter puts out. The only way to improve on this would be to have a treble boost that could raise that 4,500-7,500 curve -- that can physically be done but might not be so desirable, as it would also greatly increase the background noise.

No problem with enjoying Ancient Mary for what it is, but if frequency response is your goal you'll have to go with FM or other newer technology.

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 Post subject: Thanks Of course your correct!
PostPosted: Sep Sun 18, 2011 7:33 pm 
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All my examples were +/- 3 db. Any feelings o low end... I mean some floor models had 15" woofs. Thanks Mark


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Sun 18, 2011 7:46 pm 
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Attachment:
Philco_95_response.jpg
Philco_95_response.jpg [ 44.61 KiB | Viewed 3291 times ]
From RADIO, March 1930.


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Sun 18, 2011 10:52 pm 
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Low end response is mostly limited by speaker and audio amplifier design. Low frequencies need big speakers and big output transformers, along with enough wattage to make it all work. You simply can't get really good low response out of a smaller radio, so you're generally going to limited to higher end consoles to get much below 200 hz. Even with physically big speakers, in the 30s a lot of the designs didn't really lend themselves to deep bass response. Those solid paper cones with the limited woofer excursion probably had a fair bit to do with it along with the open back cabinets allowing some phase cancellation. Wiring one of those old radios to a more modern enclosed speaker design would probably get you better low frequency response.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Mon 19, 2011 3:13 am 
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Believe it or not another limiting factor is the Q of the IF transformers. If it's fairly high it's great for selectivity but will roll off the high end. Judging by the curves on that Philco, it looks like there might be a high Q coil in the RF amp section- note that stations lower on the dial have poorer frequency response than those higher on the dial.

As far as mitigating against such roll-off plus creating a standard which receiver manufacturers could count on, the FCC initiated a modified 75u-sec preemphasis curve for AM. Back when I was working at an AM station cluster I made sure all the audio processors were using that curve and it made a difference. I could tell when we were on our 8kHz backup loop vs. our normal T-1 program line.

Scott Todd


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 12:44 am 
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yeah, high end response is affected by IF transformers. Anyone who has had the luxury of messing with a receiver with variable IF bandwidth will know all about that. The sharper the IF response curve is, the worse the audio sounds. Communications recievers in general sound bad for music for this reason unless they have a broad setting for the IF strip to not kill the high frequency response so much.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 1:30 am 
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Alan, thanks for posting the response curves of the Philco 95.

Many years ago, I picked up a 95 in a high boy cabinet. After restoring it I was amazed at the performance of that radio! Wonderful tone and sensitivity for a radio of its age! being one of the earlier radios with AVC, Philco really pulled a winner out of their hat!

-Steve

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 2:32 am 
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Getting at least 10KHz response was fairly easy and others, including communications sets even went out to 16KHz and that was with three IF stages. Most hams and SWL's were familiar with the old Super Pros that go back to 36 and were used by the thousands in WW2.

The Zenith 1204 chassis is one example of some excellent audio as are many other high end models from several manufacturers that were even better.

Remember back in the 30's the bandwith rules were a lot different and even in later years the shortwave broadcasters were not under any restrictions.

Carl


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 2:45 am 
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Gary Tayman wrote:
The figure of 20 Hz to 6,000 Hz is about as good as it's going to get; either in the 1930's or today, with broadcast AM.

Remember, an AM broadcast channel is 10 kHz wide, which means with two sidebands you can only do up to 5,000 Hz. Of course every time I bring this up, at least ten others will chime in and tell me that you can get more. Sure, but only because there's a rolloff -- so many dB down at 7,500, just about gone entirely at 10,000, .


The ten others who usually chime in have actually worked in the industry as
FCC first class radio telephone licensed transmitter engineers only because
you keep insisting on making things up and reposting the same nonsense
over and over and over.

The 10kHz channel spacing HAS NOTHING TO DO WITH THE MAXIMUM AM BANDWIDTH.
The FCC assigns stations with a minimum 30kHz spacing if there is chance of sideband
interference.

Please cite the FCC regulations that limit AM broadcast bandwidth below 10kHz.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 3:05 am 
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Are you saying that AM transmitters put out 15,000 Hz, plus or minus 3 dB?

There is a curve, don't have it in front of me, but there are specs that show a certain attenuation at 5,000, 7,500, etc.

Whatever you insist it is, please SHOW US THE CURVE!!

Geez, isn't this what this thread is asking about in the first place?

And again I say, as has been backed up by others here, the RECEIVER limits the response anyway.

But once again, someone is asking, so I respond, and you chime in and tell me I'm wrong. So what is RIGHT?

What is the actual frequency response of one of the broadcast AM transmitters you worked with, plus or minus 3 dB (compensating for any pre-emphasis)???

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 3:16 am 
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There are no curves for frequency response below 10.2kHz.

Indeed, the 10kHz limitation is fairly recent, at the request of the NAB.
Back when music programming predominated on the AM band the bandwidth
was considerably greater.

Here is the actual FCC CFR. Do you see ANY restriction in bandwidth below 10.2kHz???
Stopband filter technology has come a long way. There is no need to start attenuating
frequency response below 10.2kHz to meet FCC requirements. The discussion is about
making up non existent FCC regulations, not the limitations of cheap AM receivers.

Title 47: Telecommunication

CHAPTER I: FEDERAL COMMUNICATIONS COMMISSION (CONTINUED)

SUBCHAPTER C: BROADCAST RADIO SERVICES

PART 73: RADIO BROADCAST SERVICES

Subpart A: AM Broadcast Stations

73.44 - AM transmission system emission limitations.

(a) The emissions of stations in the AM service shall be attenuated in accordance with the requirements specified in paragraph (b) of this section. Emissions shall be measured using a properly operated and suitable swept-frequency RF spectrum analyzer using a peak hold duration of 10 minutes, no video filtering, and a 300 Hz resolution bandwidth, except that a wider resolution bandwidth may be employed above 11.5 kHz to detect transient emissions. Alternatively, other specialized receivers or monitors with appropriate characteristics may be used to determine compliance with the provisions of this section, provided that any disputes over measurement accuracy are resolved in favor of measurements obtained by using a calibrated spectrum analyzer adjusted as set forth above.

[b](b) Emissions 10.2 kHz to 20 kHz removed from the carrier must be attenuated at least 25 dB below the unmodulated carrier level, emissions 20 kHz to 30 kHz removed from the carrier must be attenuated at least 35 dB below
the unmodulated carrier level, emissions 30 kHz to 60 kHz removed from the carrier must be attenuated at least [5 + 1 dB/kHz] below the unmodulated carrier level, and emissions between 60 kHz and 75 kHz of the carrier frequency must be attenuated at least 65 dB below the unmodulated carrier level. Emissions removed by more than 75 kHz must be attenuated at least 43 + 10 Log (Power in watts) or 80 dB below the unmodulated carrier level, whichever is the lesser attenuation, except for transmitters having power less than 158 watts, where the attenuation must be at least 65 dB below carrier level.[/b]

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 9:28 am 
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So I will ask AGAIN, what is the frequency response, plus or minus 3 dB, of the AM transmitters you worked with?

The 5,000 Hz limitation is the textbook answer. It is what my teacher told me back in school. If you feel like going after the school to take away its NATS accretidation, fine. Whether it's written as a law, or held to in real life, if you want to make a point, fine. It's obvious that there's not going to be a brick wall on this, rather a curve where the sidebands will be attenuated by a certain amount.

As for emissions higher than 20kHz, you can't hear this anyway; there simply is no audio signal in that range to begin with. Those specs are referring to the purity of the carrier sine wave, ensuring there are no harmonics or other spurious emissions that are going to bleed all over everyone else. As for the 10.2 kHz, this is a rather strange number -- why the .2? Radio RECEIVERS are designed not to pick up 10kHz, for the simple reason that any adjacent station would make it whistle like crazy. Alright, so the FCC won't assign an adjacent station in a listening area, sure. But let me tell you, any adjacent station, I don't care if it's located on the other side of the Rockies, it's going to skip its way to your radio and make it whistle. Some radios will whistle anyway, as they don't quite get that 10kHz totally filtered out.

SO -- since radio receivers, in the interest of selectivity, and in the interest of keeping that 10kHz whistle to a minimum, limit their response to around 5,000 Hz (or less) anyway, wouldn't it be in the best interest of the broadcaster to place similar limits on the transmission? After all, any emissions that aren't heard by radio receivers are simply wasted energy anyway; cut the sidebands and you'll go farther. Today especially, when radio stations are compressing their signals to the max and pushing modulation right up to 100% (sometimes even more on positive peaks), you'd think they would fine tune the sidebands for maximum efficiency. But I'm also sure that different people did things differently, so one station can vary from another.

So again, what is the frequency response of your AM transmitters, plus or minus 3 dB?

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 10:22 am 
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I remember reading a Consumer Reports test on radios from 1947 or 1948 and they connected what looked like an
AA5 to their reference speaker, an Altec Duplex, and they said the sound was very close to thier reference tuner.
I don't remember them mentioning actual frequency response, they were just showing how important the speaker
and enclosure were.


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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 12:46 pm 
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markbark wrote:
Has anyone ever measured the frequency response of 1930-1950 tube radios? What was the limiting factor, bandwidth or speakers? During the time Stereo Review actually measured anything about Am in receivers it ran the gamet from 20hz to 6000hz and as bad as 300hz to 1900 hz.


Getting back to the original comment, the fact remains that back in the 1930's consumers weren't all that interested in frequency response specs or much of anything else. Outside the lab, about the most specific comment you'd hear about a radio's performance is, "Gee, it's got a nice tone." Even phonograph records, if you got 7,500 Hz out of one of those you'd be doing darned good. And regardless of the FCC specs for frequency curves, the fact that the channels were designed for 10kHz spacing (only 9 in many countries) was evidence enough that there was little concern for anything above 5 kHz anyway. I remember reading in some older textbook that the human ear cannot detect harmonic distortion until it reaches 17% -- wow!

In the early days of FM, whose specs are fairly flat (aside from pre-emphasis) up to 15kHz, one of the big worries from many radio stations was the limitations of the transcription machines, that they were incapable of reaching this and that better technology was needed. Also during this time, as RCA was contemplating their new "Hi-Fi" 45 rpm records, their marketing agents were nervous about introducing these records, believing that the "tone" of existing players might be more appealing to consumers than the overall tone of the new high-fidelity types. Yes, since the mid-1930's there was talk of improving fidelity, but a lot of apprehension about the use of a higher standard.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 3:00 pm 
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Gary Tayman wrote:
So I will ask AGAIN, what is the frequency response, plus or minus 3 dB, of the AM transmitters you worked with?

The 5,000 Hz limitation is the textbook answer. It is what my teacher told me back in school. If you feel like going after the school to take away its NATS accretidation, fine. Whether it's written as a law, or held to in real life, if you want to make a point, fine. It's obvious that there's not going to be a brick wall on this, rather a curve where the sidebands will be attenuated by a certain amount.

So again, what is the frequency response of your AM transmitters, plus or minus 3 dB?



Gary, I showed you the FCC regulations, in black and white. You believe what you want,
and if those schools thought you 5kHz was the modulation limit they were full of crap.
Last time this story came around you insisted it was an "army buddy" who told you that
tale. Get the story straight. There are NO curves below 10.2kHz.
Federal law only specifies maximum sideband power above 10.2 kHz
period. And that is a recent regulation brought about by industry request to allow
cramming more stations into the existing spectrum.

When I was a transmitter engineer we were required to ran annual proofs of performances
from the studio equipment through the transmitters to prove the audio response wasn't
impaired. I don't have the CFR on hand that were in effect in the 1960s. Why
don't you get off your soapbox, look up the ancient regs for proofs going back into
the sixites, and give us a report? I'm sick of doing your research.

You have had several licensed engineers tell you otherwise but you still continue weaving
fables. What stations did you work for?

webquote:

The limitation on AM fidelity comes from current receiver design. Moreover, to fit more transmitters on the AM broadcast band in the United States, maximum transmitted audio bandwidth is limited to 10.2 kHz by a National Radio Systems Committee (NRSC) standard adopted by the FCC in June 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 3:13 pm 
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So I will ask YET AGAIN, what is the frequency response, plus or minus 3 dB, of the AM transmitters you worked with? I'm not talking about what the FCC allows, but what IT DOES.

I never said anything ever about an army buddy; that's a lie, I was never IN the Army. As for nine others telling me the same thing, I know of no others. Do you have names? I'm sick and tired of this lambasting. How about an ANSWER? Do you not know?

An absolute pet peeve of mine is someone who, when asked, cannot give me any information whatsoever, but if I say or do something he's the first to pounce on me to say I'm wrong.

The question, WHAT IS THE FREQUENCY RESPONSE, PLUS OR MINUS 3 dB, OF AN AM TRANSMITTER? You insist I'm wrong, what is RIGHT? Huh?

If there's no answer, there's no sense in continuing this thread.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 3:42 pm 
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Gary Tayman wrote:
So I will ask YET AGAIN, what is the frequency response, plus or minus 3 dB, of the AM transmitters you worked with? I'm not talking about what the FCC allows, but what IT DOES.

The question, WHAT IS THE FREQUENCY RESPONSE, PLUS OR MINUS 3 dB, OF AN AM TRANSMITTER? You insist I'm wrong, what is RIGHT? Huh?

If there's no answer, there's no sense in continuing this thread.


There are voluntary NRSC limits regarding "curves." Stations are free to follow them
or not, as far as I know. You were the one who suggested that
AM bandwidth is limited to 5-kHz because of the 10kHz channel assignments.
I simply pointed our there are no FCC regulations limiting AM bandwidth to 5kHz.

Here is a link to the old regulations that dealt with proof of performance back before
the NRSC changes took effect in the 80s.

http://radiomagonline.com/tutorials_tip ... dio_proof/
As I recall, going back over 35 years, the rules required measuring transmitter
frequency response to at least 16kHz, and distortion levels, at fixed intervals
to that frequency.

Regardless...
the existing FCC CFRs are quite explicit, and don't show any required reduction in
sideband power below 10.2 kHz. I really don't have to "prove" anything besides
showing the current laws and regulations that are in effect.

Here is some literature with current NRSC RF mask requirements:

http://www.thebdr.net/articles/fcc/rules/NRSC.pdf

Again, I don't see ANY required rolloff below 10.2kHz shown in the current
regulations.

Whether stations chose to bother ensuring they
have the maximum allowed fidelity permitted by FCC regulation, or having
any pride in their signal is beyond my control. Stations running IBOC
are the only ones that limit AM fidelity to 5kHz to protect their digital
sidebands either side of carrier.

Pete

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 4:09 pm 
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Quote:
Regardless...
the existing FCC CFRs are quite explicit, and don't show any required reduction in
sideband power below 10.2 kHz. I really don't have to "prove" anything besides
showing the current laws and regulations that are in effect.

Whether stations chose to bother ensuring they
have the maximum allowed fidelity permitted by FCC regulation, or having
any pride in their signal is beyond my control. Stations running IBOC
are the only ones that limit AM fidelity to 5kHz to protect their digital
sidebands either side of carrier.

Pete


Thank you, finally, that DOES answer a lot of questions I have. So in other words the current spec, because of IBOC, is 5kHz for stations using it, and other stations have output responses that vary according to the design/adjustments of the transmitter. I would still think that many indeed taper off above 5 kHz, but again it's up to the guys running the transmitters.

But what is the significance of the 10.2 kHz? That one is a mystery to me, as the carriers are 10kHz apart. Again, many receivers are designed so as to minimize the 10kHz whistles that can occur especially at night. So why would the transmisster spec want to go just beyond that, unless it's just to allow for rolloff if a transmitter wants to get close to it.

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 Post subject: Re: Frequency response of 30's radio
PostPosted: Sep Tue 20, 2011 4:30 pm 
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Alan Douglas wrote:
Attachment:
Philco_95_response.jpg
From RADIO, March 1930.


Alan:

Thanks.

You not only answered the OP, but you now have me wanting to pull my Philco 95 out of the to do later when I can group and move it up to the to do next group. I didn't realize that it had the potential to be such a nice performing radio!

BR,

Terry


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