Spectrolite displays real-time, constant-Q audio spectrograms.
A spectrogram is a way to visualize the frequency content of a sound or other signal as a function of time. In Spectrolite, time is shown on the horizontal axis and frequency on the vertical axis. Brightness corresponds to the signal amplitude, or sound volume.
In Spectrolite, the frequency axis is logarithmic rather than linear, as this corresponds more closely to the human perception of audio frequencies. It is also well suited to visualizing music, as each octave or other musical interval is always the same width.
The "Q" in constant-Q refers to the "quality factor" of the filters used to extract the frequency bands displayed in the spectrogram. In a constant-Q spectrogram, the bandwidth of each filter is proportional to its center frequency, which makes Q constant across the frequency range.
Thanks to the constant-Q property, signals will appear as equally sharp traces in the spectrogram regardless of their frequency, even though the corresponding bandwidth varies according to the logarithmic frequency scale.
When Spectrolite starts for the first time, it will ask for permission to use the microphone. If you don't allow this, Spectrolite won't be able to hear anything, and the display will stay black. If access to the microphone was denied when Spectrolite was started the first time, you can still grant access later in the Settings app under Privacy / Microphone.
To check that Spectrolite is working, try whistling; you should see a trace on the display that rises and falls with the pitch of your whistle.
At the top left corner is a button that opens a set of controls including a Gain slider. The Gain slider controls the overall brightness of the spectrogram display. If the display is all black, move the slider to the right to increase gain and bring out the signal. If you are in a noisy environment, the background noise may drown out the signal and make much of the display white; if that happens, move the slider left to reduce the gain.
The spectrogram initially grows towards the right, but when the edge gets close to the right edge of the screen, it will automatically start scrolling so that the most recent audio stays visible on the screen. If you want to look at older audio, you can swipe on the display to go back in time or forward again to the present. You can also pinch to zoom in time, frequency, or both.
Spectrolite keeps a limited amount of recorded audio in memory. This is usually about two minutes, but may vary depending on the audio sample rate of your device. If you let the display constantly auto-scroll such that the most recent audio is always visible, the oldest audio is automatically erased so that the scrolling display can continue to show the newest audio indefinitely. If you swipe back to view older audio, as long as you don't swipe too far or stay there too long, recording will continue uninterrupted, but if you swipe back by more than about a minute or swipe back and stay there, the recording will automatically stop. If you then swipe forward to the present again, recording will automatically restart, but the recording will have a gap.
Spectrum analysis involves a fundamental trade-off between time and frequency resolution: the higher the frequency resolution, the lower the time resolution, and vice versa.
In a constant-Q spectrogram, the frequency resolution is constant over the whole frequency range when measured in logarithmic units such octaves, semitones, or orders of magnitude, but it varies when measured in linear units such as Hz. The resolution in Hz is highest (or in other words, the bandwidth is narrowest) when analyzing low frequencies, for example around the lower edge of human hearing at 20 Hz.
Because the frequency resolution in Hz varies by frequency, the time resolution also varies by frequency: high frequencies have a higher time resolution than low ones. This can be demonstrated visually by making a short, impulse-like sound, for example by tapping on the device with a fingernail. This will produce a characteristic horn-like shape that is sharpest at the top (high-frequency) end where the time resolution is highest, and flares out exponentially towards the bottom (low-frequency) end where the time resolution is lower. This shape is also the basis of the Spectrolite app icon.
You can control the overall trade-off between time and frequency resolution by changing the value of Q. This is done by moving the Q slider. Moving the slider to the right increases Q, which increases the time resolution and decreases the frequency resolution. Visually, this has the effect of making the spectrogram sharper in the vertical direction and blurrier in the horizontal direction. Moving the slider to the left has the opposite effect.
If you have purchased the Pro features, you can enable time, frequency, and amplitude scales. The time scale shows the time since the start of the recording, either in seconds or in hours, minutes and seconds. The frequency scale can show either semitones (labeled by their note names) or Hz.
When you zoom the display, subdivisions of the time and frequency scales will automatically appear and disappear to keep clutter under control. For example, the time scale will initially show divisions of 10 seconds, but as you zoom in, 1 second and 0.1 second subdivisions appear. The note scale will initially only show octaves, but as you zoom in, semitone subdivisions appear.
The amplitude scale appears at the bottom of the screen. It contains a horizontal trace similar to the spectrogram trace of a sine wave of increasing amplitude, labeled in dbFS. To estimate the amplitude of a frequency component in the spectrogram, find the part of the line having the same brightness and width, and read the amplitude off the dBFS scale. The dBFS unit stands for decibels relative to full scale, where 0 dBFS corresponds to a sine wave of the greatest amplitude that can be digitally represented without clipping. Since most signals are of lower amplitude than this, the dBFS numbers are usually negative.
The View control lets you switch between the spectrogram view and a spectrum view, where frequency is shown on the horizontal axis and amplitude on the vertical axis. Like the spectrogram view, the spectrum view supports touch panning and pinch zooming in both the horizontal and the vertical direction. The Q factor can be adjusted and frequency and amplitude scales can be selected using settings shared with the spectrum view.
Like the spectrogram view, the spectrum view is based on constant-Q analysis, where the bandwidths of the analysis filters scale with frequency. This means lower-frequency bands will have narrower bandwidths, and consequently, they will respond more slowly to changes in the signal than the higher-frequency bands do. You can see this by tapping on the device; this will result in a "wave" moving from right to left as the higher-frequency bands respond first and the lower-frequency bands take successively longer. This effect is most noticeable at high values of Q.
When the signal includes random noise, the spectrum can fluctuate quickly, particularly in the higher frequencies where the analysis filters respond the fastest. To calm the display, use the Damping slider, which applies an adjustable amount of time-averaging to the spectrum. A setting of 0 gives the fastest response, and larger values make the spectrum change more slowly.
Note that in a constant-Q spectrum, a white noise signal will show a rising slope of 3 dB per octave, and the spectrum of a pink noise signal will be flat.
By tapping the Share icon you can share, save, or print the current spectrogram or spectrum view.
If your device is running iOS 14 or newer, you can change the colors of the spectrogram, playhead, and scales (if purchased), by tapping Preferences and then one of the color wells to the right.
In addition to the built-in microphone, Spectrolite also works with the headset microphone and with other compatible audio input devices, such as compatible USB audio interfaces connected via a Lightning to USB Camera Adapter. Spectrolite uses the default audio input, which typically is the most recently attached audio input device. Even if your audio interface supports stereo or multi-channel audio, Spectrolite displays the signal as mono only.
If you have any problems with Spectrolite, please email firstname.lastname@example.org. If it's working fine, please write a review in the App Store.
Spectrolite is Copyright © 2019-2021 Waxing Wave.
Spectrolite is powered by the Gaborator constant-Q spectrogram library.