Introduction to eSSB Audio
Hi-Fi Audio Processing
Introduction to eSSB Hi-Fi Audio Processing
NU9N SSB / eSSB Hi-fi Audio Introduction
eSSB SSB Hi fi, Mid fi and Low fi audio for Amateur Radio that increases the intelligibility and reliability of communications audio is my primary focus for designing this web site.
It is my desire to be of some assistance in providing some fundamentals regarding good quality SSB audio transmission and reception. Of course, there are several variables that will determine
the outcome of your SSB audio quality. Also, "Good quality" is a very subjective term determined by several factors, such as your hearing attributes, the transceiver being used, your transceiver bandwidth, the headphones being used (if
any) and your personal tastes.
For our purposes here, I am going to define the medium or "Mid-Fi" threshold of "High Quality Extended SSB Audio" as being audio with very low IMD and Harmonic Distortion
properties that is relatively "FLAT" within a given bandwidth of 50Hz ~ 4kHz or more with audio amplitude descending linearly by about 1dB to 2dB per Octave (depending on the bandwidth being used) as we move up in frequency. The reason
I picked a 2dB/Oct slope was to make up a bit of perceived high frequency response that is lost in this bandwidth. If we were working within a 3kHz bandwidth, I would choose a 1.5 dB/Oct roll-off since broadcasters in 9.5kHz bandwidths
and above use a 3dB/Oct roll-off. 3kHz bandwidth, being 1/3 of this, would then require less of a roll-off (by 1/3) resulting in 1dB/Oct. Obviously, some of this has to do with some psyco-accustic perceptions of what and how we hear.
A sound that would produce a graph with equal amplitude across the frequency spectrum would sound very brittle! Great for DX, but not so pleasant for extended and pleasing listening!
See the "eSSB" Page for a complete definition of eSSB and it's purpose.
The psychological magnitude loudness is associated with a given SPL. Judgments of whether two sine tones sound equally loud show fairly low dispersion among different individuals.
Judgments on "how much" louder one tone is than another require previous conditioning or training and yield results that fluctuate greatly from individual to individual and from occasion to occasion.
Curves of Equal Loudness and the "Fetcher Munsen Curve"
Tones of the same SPL but with different frequencies are in general judged as having different loudness. SPL is thus not a good measure of loudness, if we inter compare tones of different
frequency. Experiments have been performed to establish curves of equal loudness, taking the SPL at 1 KHz as a reference quantity. These are shown below...
This shows the relationship between volume and frequency response of your ears, or what does the "Loudness" button do. For example, at 20 Hz, the lowest SPL you can hear is about 65 dB. At 500 Hz however you can hear all the way down
to 0 dB. Notice that around 3kHz is the most sensitive region. A Phon is a unit of volume that roughly corresponds with dB SPL.
The chart below demonstrates the threshold of hearing at different decibel levels across the frequency range of human hearing. As decibel level increases, the frequency response of our ears begins to flatten. This is known as the Fletcher-Munson
The characteristic we identify with the shout is the power behind it, the force used to make the sound travel. When we measure something in decibels, we take the ratio of the power of the measured
sound (P) to the reference-level power (Pr) of 10-12 watts per square meter. The actual decibel reading is mathematically expressed 10 log10 (P/Pr). You can add and subtract decibels, but you can multiply or divide them. Since not all
frequencies are perceived to be as loud to the human ear, even at equal power, most decibel meters attenuate to the specific frequencies while making a measurement. Twenty decibels is not twice as loud as 10 dB, but 10 times as loud.
To double the perceived loudness of a violin, you need an additional 8-9 violinists. That's one reason why a modern symphony orchestra has as many members as it does, to achieve a balanced volume.
The graph below represents a sloping plot of about -3dB/Octave:
This graph would be produced by what I consider to be ideal "sounding" audio in this bandwidth, while still being relatively narrow compared with that of an Amplitude Modulated (AM) signal. Notice
the gradual downward slope as frequency increases. This is normal and is theoretically the characteristic of perceived flatness such as "Pink Noise". Again, if the graph had been a straight line all at the same amplitude, the perceived
"sound" of it would be very brittle and tinny such as "White Noise".
This next graph displays a comparison of 5 different types of transmission;
Pink Noise / AM Broadcast / eSSB (W1) / eSSB (N2) / Normal and DX Audio
These lines look a bit confusing, but as you can see, there are some major differences in these graphs and in the sounds of each audio source that produced them.
Pink Noise is perfectly and perceptibly "Flat" (linear as opposed to logarithmic) descending 3dB/Oct., and spans from 20Hz ~ 20kHz. AM Broadcast (White) has about a 9.5kHz audio bandwidth (that's a 19kHz RF bandwidth) and is relatively
flat from about 60Hz ~ 9.5kHz with roll-off occurring below and above this. The Red line was yours truly on my Kenwood TS-850S / DSP-100 with some processing. The Cyan graph was made while listening to a 3.5kHz (N2) SSB station.
And the Yellow graph was made while listening to one of the prominent DX'ers on the 20 meter band which is more common for SSB than not.
With a few fundamentals being presented, this site was designed to help you achieve a sound that would produce a graph similar to GRAPH 1 for both transmit and receive. If I can be of any assistance in any way with your setup,
send an e-Mail to: John@nu9n.com
Please see "SSB RX Setup" and "SSB TX Setup" for more details and explanations.
My Philosophy About SSB Audio
There are many philosophies among the Amateur community regarding SSB audio and mine basically boils down to two... "Rag-Chew Audio" and "DX Audio". This site is exclusively dedicated for the "Rag-Chew"
type of audio! My personal SSB audio manifesto is this:
The production of clean, high quality, relative full fidelity SSB audio within a reasonable Amateur Radio passband width, required for this fidelity, not to exceed any bandwidth
required for good Amplitude Modulated RF.
The exploration and experimentation of both old and new audio technology and of the equipment needed to accomplish our goals while staying within our individual financial budgets.
The promotion of such experimentation within this perceived high fidelity audio
domain without being an offense, or causing malicious interference to those who do or do not participate, while abiding to FCC Part
97 rules and regulations, as vague as some of them may be.
The desire and accomplishment of pushing the envelope of excellence.
The establishing of lasting friendships in which those who are interested in quality modulation can share in the richness of the experience.
The enhancement of technical skills through experimentation of such an endeavor which is, after all, one of the mandates of Amateur Radio.
All information given on this site are the express opinions of NU9N and I assume no responsibility
for the outcome produced by such suggestions and opinions that may cause band splatter, distortion and/or unacceptable transmitter I. M.
There are those who will profoundly disagree with the contents and philosophy of this site, regarding the purposes of what the amateur radio
service was intended to accomplish.
However, it is my contention and desire to enhance the experimental aspect of the hobby while keeping with FCC rules and regulations and
being a service (nonprofit of course) to those interested in producing exceptional narrow-band and extended-band high quality SSB audio.
Every amateur radio operator is responsible for his or her signal quality and compliance with FCC standards, as vague as they may be in some
All contents of this site, including text, graphics and MP3 audio recordings, are considered the property of NU9N and are protected by copyright
law and may not be reproduced or distributed in any way or form, without the express written consent of NU9N.