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I have noticed this question has been raised more than once on this forum. As a person who grew up in the broadcast industry, and is currently a RF electronics design engineer I hope this shed some light on this subject.
The reason why FM broadcasting standards use 30Hz for the low end cutoff point is based on three major points.
1. From the early days of broadcast transmitter design to post modern times (mid 70’s), all audio inputs to tube type exciters (and some transistor) was done as a balanced or differentially driven input (much like today with XLR connections). This type of audio drive required a coupling transformer to convert balanced input to unbalanced output (single ended) and to isolate the tube circuitry high voltage potentials on the exciter side from the audio line amplifier driver. Transformers are notoriously lossy as the frequency goes down largely due to increasing eddy currents in the magnetic core material, core material saturation potential, and I squared R power loss in the windings. Because of these effects the transformer automatically acts like a high pass filter. In addition the transformer size would have to increase many fold to accommodate a core size that could pass 10-20Hz frequency energy with little or no loss. Practical economics become a major deciding point by the transmitter manufacturers on size and cost for marginal sonic improvement over a lossy transmission medium. Plus the receivers on the other end were not designed to faithfully reproduce (nor capable of) this sub-sonic energy. Sub woofers are a relatively new thing to the modern audio world. I don’t recall anyone mentioning booty bass back in the 1940’s 50′ or 60’s (and even earlier). Today with modern audio electronics the use of coupling transformers has been replaced by active (op-amp) servo-balanced input circuit designs that can pass DC to 20kHz (or higher) with no loss. But even so the 30Hz standard is still applied.
2. Before the advent of CD’s and magnetic tape, vinyl records were the recording medium of the day. As most know, the creation of a vinyl record is a purely mechanical process. Because of this the mastering machinery imposes a physical low frequency limit governed directly by the mechanical travel distance of the cutting head. 30Hz or less energy causes excessive cutting head deflections using up way too much of the allotted groove area per revolution. This has the compound effect of shortening the record playing time for little sonic gain. Your LP (long play) would become a SP (short play). Plus typical consumer grade playback styli and cartridges were too stiff to deal with these low frequency excursions and the sub-sonic energy becomes indistinguishable from turntable rumble. So the recording industry (RIAA) imposed a low end frequency cutoff point during record mastering that is directly carried over into broadcasting by virtue of the source material’s built in sub-sonic limit. So why design AM or FM exciters to transmit sub-sonic energy that is never there in the first place? Today’s CD’s don’t have these limitations. You can essentially encode DC onto a CD if you really wanted to (but they don’t for other reasons).
3. Early FM exciter design used primitive frequency control methods to keep the transmitter on channel. Sub-sonic energy (30Hz or less) caused exciter control instability making the detected signal at the receiver sound like it was a motor boat. Government regulations also required a broadcast signal to stay within a required frequency tolerance, and with a motoboating exciter it was easy to violate the regulations.
But as technology improved (advent of the PLL) this became less of a problem. Today’s carefully engineered modern PLL exciters can maintain frequency control even with sub-sonic energy present (note the phrase "carefully engineered" !). There are some professional exciters claming modulation all the way down to DC. Of course transmitting DC is pointless from a listener perspective and violates government frequency tolerance specifications, but it is a good marketing tactic to demonstrate the low end booty bass capability of the exciter. But in all fairness do you the radio listener honestly think those iPod style earbud head phones can faithfully reproduce 10-30Hz let alone DC?Cheers
Sparky
Thank you, Sparky!
EXCELLENT explanation.
I’d also like to add a 4th point:
Even though our ears can hear down to 20hz, sensitivity decreases with frequency. Very few systems can produce it (most subwoofers do not go much below 40), and the vast majority of music — even booty bass — does not go below 40 for this reason alone. Why put the audio in if it won’t be heard? 30 is an extremely low frequency, and already requires a tremendous amount of amplitude to be audible. Going below this, we’d be wasting modulation on something that will not be audible at all for most listeners, limiting our maximum volume and adding distortion.
Sort of off-topic, but I believe the whole fallacy is rooted in our tendency to fixate on numbers. Does it do 20-20000hz? Is the processing 24-bit? How can a modern 1.6 GHz Celeron be faster than a 3.0 GHz Pentium 4?
Numbers are easily quantifiable, but reality is just not that simple.For example, it would be perfectly possible to have a 192 kHz audio processor with 24-bit resolution and 5 – 40000hz frequency response, sound absolutely awful. The numbers simply measure specific parts of the processing, but in the grand scheme of things they are basically meaningless.
People generally do not understand just how much dynamic range 16-bits really is. In a properly dithered 16-bit signal, the noise floor (as viewed on a spectrum analyzer) is at -120dB. A single sinewave at full scale will read at 0dB, and a squarewave will read 2.1dB. That’s over 120dB of dynamic range!
So, how much is 120dB? Actually, it comes out to a gain of over one million. After recording the strongest sound possible in 16-bit, you can attenuate it by a factor of a million before it drowns in noise. Concerto for pin-drop and air horn? No problem!
Regarding sampling rates:
44100hz is enough to *accurately* reproduce any waveform 20-20000hz, and even a little above, depending on what filter is used. Amplitude, and phase will all be completely accurate within this band. It does not matter that the playing connect-the-dots on the sampled audio looks nothing like the original waveform — it’s the reconstructed waveform (after upsampling or D/A conversion) that matters.
If we make the comparison to 192000hz/24-bit (DVD-Audio maximum quality).. Let’s say we start with a full scale 400hz tone, but add a tiny bit of clipping somewhere in the chain. Let’s say 1 percent overload.. This adds distortion sideband up to -60dB! This single percent could be argued to have degraded our 24-bit audio to nearly 8-bit resolution. All it takes is a tiny little mistake like this, to completely undo any perceivable benefit of the higher resolution. Wouldn’t it have made more sense to spend the available finite resources on better algorithms for the lower resolution audio, instead of wasting it where it does not make an improvement?
In short, I’ll take well made 44100hz/16-bit (CD quality) over poorly made 192000hz/24-bit, any day.
Also, for those who still want frequency response below 30hz, the high pass filter does have an off-switch. 🙂
///Leif
Leif,
Good points… all of them.
I might add the only known creatures that can truly "hear" 30Hz and lower frequencies are whales.
The only known musical instrument that can truly produce 30Hz note is a pipe organ, usually requiring massive amounts of air and a hefty compressor to pump it.Sparky
I may be wrong here but I don’t believe elephants truly hear 30Hz. They sense it much like we do.
Whales on the other hand can hear 30Hz or lower largely because of the excellent transmissive properties of water (approx 1500m/sec).
Low frequency energy travels great distances in water due it’s elastic nature, and evolution has given whales "ears" that are perfectly developed to "hear" these frequencies.
You don’t hear whale love songs sung in soprano for a reason 😉I do hear 30 hz.. Down to about 25, actually. I say this with confidence, because 25hz still sounds like a musical note.
The last time I tested this was when building the stereo system in my 2001 Chrysler Concorde a few years back.
The subwoofer consisted of four 12" rockford-fosgate subwoofers, isobarically mounted (clam-shell) in two pairs, in a three chamber ported enclosure. The box took more than half of the huge trunk, and consisted of two separate enclosures (ported towards the center chamber, ports tuned to 21hz) with one end of the center chamber having a big hole, lined up with the ski-hole in the back seat. I used a rockford fosgate class D amplifier (1500 W according to the box, probably at least 1000W RMS in reality).
This sub played down to 21 Hz, with resonance at 28. It’s the deepest, cleanest bass I have ever heard from a car 🙂.
Here’s some photos: http://leif.cx/photos/concorde/
Of course, even with this setup, it was rather disappointing how few songs make use of it. Having 30hz bass notes is extremely satisfying when you have a system that can produce it. Booty bass, contrary to popular believe, doesn’t really go below 50. Remember, it’s designed to be played loud through cheap band-pass enclosures, with underpowered amplifiers 😉.
Anyway, I figure, if I can, elephants probably can too. Don’t they have infrasonic mating calls?
///Leif
[quote author=”Leif”]Anyway, I figure, if I can, elephants probably can too. Don’t they have infrasonic mating calls?[/quote]
Easily confused with bowel movements.[quote author=”JesseG”]Easily confused with bowel movements.[/quote]
So might the 25 hz tones we placed in the left channel at the end of our source music to signal to the automation to start the next source. If that were not cut, but broadcast on the FM station, listeners might think elephants were in distress.
I don’t know if the 30 hz cut was made to prevent the automation tone from being broadcast, or the automation tone was 25 hz because the audio signal was cut at 30 hz.
Traditional subsonic filters weren’t very sharp. Even an 18dB/octave high pass filter (with the -3dB point at 30hz) would likely only suppress 25hz 6dB below 30hz. So, the automation tone would probably still have been audible to anyone who has a subwoofer that can play that low. Not a very common situation though 🙂.
The ones in BBP are a little different. They’re extremely sharp — with the 30hz filter enabled, 25hz is at least 40dB down!
///Leif
[quote author=”Leif”]Traditional subsonic filters weren’t very sharp. Even an 18dB/octave high pass filter (with the -3dB point at 30hz) would likely only suppress 25hz 6dB below 30hz. So, the automation tone would probably still have been audible to anyone who has a subwoofer that can play that low. Not a very common situation though 🙂.
The ones in BBP are a little different. They’re extremely sharp — with the 30hz filter enabled, 25hz is at least 40dB down!
///Leif[/quote]
Having worked for stations in the 80’s that used automation, I can say it was possible to hear the 25Hz tones on-air even on a typical stereo system of the time. The tones were filtered out, but just enough got through so you could hear it especially if the audio processing was raising low audio levels.There’s a "beautiful music" internet station I know of that obviously got their songs from old automation music reels. The 25hz tones sound unfiltered and are quite audible. So it seems possible to stream 25Hz also!
We are using an old "PT31" for switching purposes.
With BBP @30Hz cutoff the Bittone isn’t audible onAir anymore.
Cheers Tom
Wow… totally forgot about the old automation tones. Haven’t thought on those line in quite awhile.
On a different note I will add that subsonic tones were used in some instances for remote transmitter metering information sent on SCA subcarriers. the 10-25Hz control tones were commonly mixed with the Muzak program audio. Most SCA receivers on the subscriber end had high pass filters that largely filtered this out. But between music selections and the quality of the receiver and signal received, you could sometimes faintly hear the warbling of the control tone in the grocery store. I forget which transmitter manufacturer used this system (Marti, McMartin, Continental?). This sub-sonic tone system seems so rudimentary compared to today’s high precision digital systems. But it sure was reliable from a reception standpoint.
Interesting!
Does anyone still use SCAs?
I believe it might be possible, at least from a technical standpoint, to generate a 67 kHz SCA in software, make it part of the regular Stereo MPX signal, and output it through a sound card 🙂.
///Leif
[quote author=”Leif”]Concerto for pin-drop and air horn? No problem![/quote]
"Concerto for pin-drop and air horn"… I swear, that is the funniest damn thing I’ve heard this month. I’ve already worked it in to a joke at the office.HA, thanks Leif…
[quote author=”celar”][quote author=”Leif”]Concerto for pin-drop and air horn? No problem![/quote]
"Concerto for pin-drop and air horn"… I swear, that is the funniest damn thing I’ve heard this month. I’ve already worked it in to a joke at the office.HA, thanks Leif…
[/quote]The pin drop would only be represented by the LSB (or a fraction of the LSB more likely), but… hey, it’s a pin drop.
Now a shuttle launch on the other hand… is something that you can’t even get close to replicating properly in the analog world yet, much less digital. 😆 It’s over 180db of dynamic range, with frequency response from near DC coupled, up to hundreds of kHz.
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