The Audio Domain

Human Voice Reproduction

Human voices, including bass, baritone, tenor, alto and soprano, occupy the range from 80 Hz to 1,050 Hz for vocal chord reproduction and higher for vocal harmonics and sibilance, to 10,000 Hz.

Therefore when selecting a voice microphone a frequency response of 80 Hz to 10,000 Hz would fit nicely.

Voice recording does not require octaves 1, 2 or 10.

Actually, if your modulating signal is 10 kHz the basic bandwidth of your transmitted signal will be 20 kHz.

Used to be AM stations had to keep the audio at or below 5 kHz but seems to have been relaxed over the years.

I've never heard of a 5 khz audio requirement, but in the world I grew up in it was considered that audio out to 15 khz was acceptable, though some stations narrowed it on both bass and treble sides to get more 'power bandwidth' in the mid-frequencies, to 'fit through' the smaller speakers on portables and pocket radios.

Things change though, there was a tidal push for better fidelity in general as FM started to catch up to AM in the 1970s, stereo systems got bigger speakers at home, and the build quality of AM transmitters was improving over transformer modulation and tubes, and the upcoming push for stereo AM.

AM stations started to have flatter, cleaner audio, especially some of the smaller stations that were trying to compete on new or alternative music. With crystal detectors hooked to my stereo's line-in, I heard some stations with sound rivaling FM and it surprised me.

Later I saw NRSC 10 khz audio was being proposed, and after thinking about it, it seemed bad. There were some grumbles about the 10 khz low pass and having to buy the compliance equipment for it, but most just rolled over for it.

I think NRSC is ugly, because broadcasters fidelity, lost 10 khz of bandwidth real estate (important for future digital being discussed even then). It sent further signals to radio manufacturers and designers that AM was being degraded, and they should no longer try to offer high fidelity receivers, even if technology would evolve to allow it(noise blanking, sideband selection, low distortion detectors), which it certainly has.

You can still build a crystal radio for local stations, but the current best option for hi-fi AM is probably a software radio on a PC. With SDR you can tune on the screen, and freely adjust bandwidth to what the station is using.

SDR's should be a bigger part of the Part-15 fandom as far as I'm concerned!

Boomer

Yay Carl! Most AM radios have narrow bands, making it hard to hear fully what you're broadcasting. You can do fairly well, especially with an analog radio by tuning across the signal, and some would say that's the same equipment that listeners would have.

SDR can allow you to hear the whole signal bandwidth, your station and others, and compare them, plus record, and do spectrum analysis.

Boomer

I like radios like the GE Superadio with selectable bandwidth, it works on the GE, but probably isn't the best implementation. I'd have to read how AM-wide is done in the GE again, but I thought it's done by detuning the IF stages.

The best way to do it in analog radios could be to have two dedicated IF paths selected by the switch, then they could each be optimized. The main bandwidth-determining part is the ceramic filter used, and it shouldn't be too hard to switch narrow and wide filters in and out. CFs come in bandwidths up to 35 khz at the IF of 450 khz, so you could get 15 khz audio through one, with sharp cutoff outside the bandpass.

To get all the audio from stations today, radios could use a 20 khz width filter, for 10 khz response, with a notch filter at 10 khz and a correct de-emphasis, and that could be a great sounding radio. It just takes real designers to work on doing it, and I think today's circuit designers should be sent back to school, especially the DSP guys making today's digital radio chips, they're about the worst we've had I think, but with the potential heights digital signal processing can reach, tomorrow's radios could be the best we've heard on AM.

Boomer