HD VoIP solutions for a new home
drgeoff:
Even G.711 is not a true common denominator as there are two codecs, A-Law and mu-Law. Each is used in large parts of the world. Transcoding is necessary where they meet though it is a relatively simple operation.
For the record, before the introduction of PCM and Time Division Multiplexing there was very significant use of Frequency Division Multiplexing in which baseband analogue voice signals were single sideband modulated and stacked on carriers at 4 kHz intervals. The complete signal would then be transported over coaxial cables or microwave links.
SeanTek:
Quote from: drgeoff on December 30, 2015, 05:17:37 am
For the record, before the introduction of PCM and Time Division Multiplexing there was very significant use of Frequency Division Multiplexing in which baseband analogue voice signals were single sideband modulated and stacked on carriers at 4 kHz intervals. The complete signal would then be transported over coaxial cables or microwave links.
Got it. And FDM necessarily implies that the original analog voice signals are frequency-limited to a predetermined range, in this case, the range common to most of the human voice.
Prior to the introduction of these PCM and FDM techniques, there literally was one (effective) electromechanical circuit per phone call from one end to the other. I guess that would be up through the mid-1960s or so. Was audio quality markedly better, worse, more wide-band, or just different, back in those days? For those who remember, anyway. :)
It stands to reason that if it's just two copper wires, what really matters is the quality of the telephony equipment on either end of the line...no different than if you connected a mono ⅛" headphone jack over RJ11-style wires (modulo resistance and power issues, which would have to be matched). But for any telephony system operated by a common carrier, there would be power injectors, filter banks, surge arrestors, and all sorts of other equipment that would change the voice quality.
SteveInWA:
I don't know where you live, or how old you are, but in the good old days of copper loops and mostly analog equipment, call quality varied considerably, based on all the many factors that can impair any analog signal, just like the wide variation in sound quality of FM radio before it was available in digital format. Even under the best circuit conditions, the telephones themselves weren't designed to reproduce "high fidelity" frequency bandwidth. Remember, the microphones used to be made out of carbon ::). The Bell system developed a standard to test and rate call quality, the Mean Opinion Score, or MOS. It was an opinion score, because they had a panel of listeners rate the call quality, which, after all, was the goal: reproduce "good enough" human voice. The MOS for VoIP is now a calculated value that includes jitter and packet loss. Read more about it here:
https://en.wikipedia.org/wiki/Mean_opinion_score
A typical "land line" phone call now sounds much like it did decades ago, in terms of bandwidth, but without the static, interference, crosstalk, etc. Thinking back to the first digital PBX phone systems in office buildings, I remember how amazed we were over how quiet and clear the calls sounded, and we never thought "hey, this is narrowband".
G.711 is designed with a maximum MOS of 4.1. I haven't seen this measurement applied to WB audio. Let's not get into one of those audiophile debates over whether digitized audio sounds worse than analog audio, ok? Suffice it to say that most people are now so accustomed to the highly variable, and often miserable sound quality of mobile phone calls, that most younger consumers don't think much about it...if they're even still talking on their phones instead of texting and playing games while they jaywalk in front of traffic.
This has gotten pretty far off-topic to your original goal, which was selecting telephone equipment for your new home. Suffice it to say that any of the OBi phones would be fine choice, since they support both narrowband and wideband audio, from both a digital standpoint (selection of CODECs) and from an analog standpoint (audio circuitry engineered to reproduce wideband audio). Being firmware-based, if some future WB audio CODEC came along, it wouldn't be very difficult to upgrade the devices to support it.
drgeoff:
You might find https://www.theitp.org/historic/Short_History interesting. Although the details and dates are specific to the UK, the general principles and techniques were fairly universal throughout the world.
SeanTek:
Quote from: drgeoff on December 30, 2015, 10:47:41 am
You might find https://www.theitp.org/historic/Short_History interesting. Although the details and dates are specific to the UK, the general principles and techniques were fairly universal throughout the world.
Thank you! Yes, the read was interesting.
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