By changing the treble and bass after the fact, has no effect on the frequencies that have been received and interpreted by the circuitry of your HDT1. The display is showing the ouput frequecies of the tuner. Or better stated the bars correspond to what has been received and presented to the output.
They have centers corresponding to what we humans can hear. The lowest being around 20 hz, and the highest 20 Khz. There was a post from Brian Beezley stating that he tested the output of each bar and they were centered at, 57 hz, 77hz, 122hz, 263hz, 528 hz, 1055 hz, 2070 hz, 4180 hz, 8370 hz, and the final and higest frequency center to be 15,450 hz. each bar about 4 to 5 db.
When feeding this frequency range into your amp or receiver which is an analog device, it should then deliver these frequencies to your ears via the speakers. When your tone controls are at their center position, or flat. and your room is acoustically flat, and your ears are performing like you were 20 years old, you should hear all the frequencies displayed. That is if Sangean is delivering what it has detected in the input to the output.
But when one of these conditions is not up to par, we add a little treble to boost the high frequencies. This will not change the display, because it is happening in the amp, not the tuner. Maybe we like a real low bass, we turn the bass up a notch and the lows are boosted in the room, not on the display which remains the way the artist intended it to be. (Hopefully not the way the radio station engineer intended it) Standard analog fm engineers have boosted the signal at both ends for years to give announcers a booming voice, and to give highs what they needed to be heard in a fm system that did not accurately represent the high frequencies.
The Fine Sangean HDT1 is showing you what it received from the broadcast. If the station engineer is intentionally boosting the high end and you could see this fast, you could possibly detect spikes in the last couple bars. I doubt if our eyes are that sensitive. The best use for the graphic display would be in trouble shooting. If you see plenty of movement in all the bars you should be getting a pretty even sound. High notes are playing, right bars are bouncing. low notes are playing left bars are bouncing. Someone is singing, middle bars are doing their thing.
One good use for the graphic display is to compare stations. A station doing a good engineering job is having a good full display, This full display is only seen when there are plenty of harmonics to work with, like pop music or a full orchestra. When a solo artist is singing we should only see bars associated with the tone of his or her voice. A good test for your graphic display occurs when your local station does a test of the emergency broadcast system. The 1,000 hz tone should move the bar associated with or centered at 1055 hz. The others should be relatively flat. Stations not compressing or pumping the highs will also show a good even spread or bar movement. I never saw a graphic frequency display in the tuner end of a device. Another great idea from Sangean. It lets us see what we should be hearing. If everything is a bouncing and you don't hear it. It is probably time to invest in some new speakers, or at least replace the speaker drivers. Still didn't fix it. Time to change the battery in the old miracle ear.
Useful display, and according to Mr. Beezley, is accurate. Welcome to the digital age, where software determines what you hear and see!