What aspect of an analog slow-scan television signal encodes the brightness of the picture?

Analog Slow-Scan Television (SSTV) uses frequency modulation of an audio tone to transmit image information. Specifically, the brightness of the picture (luminance) is encoded by shifting the frequency of this audio tone. Usually, lower frequencies (around 1500 Hz) represent black, and higher frequencies (around 2300 Hz) represent white. Therefore, it is the tone's frequency, not its amplitude, that conveys the brightness levels. Distractors involving amplitude are incorrect because analog SSTV relies on FM audio tones sent over an SSB transmitter. This makes the image data highly resistant to amplitude variations caused by fading (QSB) on HF bands.