A goniometer, often used in audio engineering, provides a visual representation of the stereo image or stereo field of an audio signal. It helps engineers and producers to assess the stereo spread and phase correlation between the left and right channels of a stereo audio signal. Here's how it works in detail:
### 1. **Basic Concept of Stereo Information:**
Stereo audio consists of two separate audio channels, **left (L)** and **right (R)**, that together create a perception of spatial sound. The goniometer visually shows how these two channels interact with each other in terms of:
- **Amplitude balance** (difference in levels between L and R channels).
- **Phase relationship** (timing differences between the channels).
- **Stereo width** (the perceived distance between sounds in the stereo field).
### 2. **The Goniometer Display:**
The goniometer display is usually an oscilloscope-like screen, which uses the two channels of a stereo signal to plot a visual pattern. The X-axis represents the **Left channel (L)**, and the Y-axis represents the **Right channel (R)**. The resulting plot shows the relationship between the two channels in real time, producing a constantly evolving shape.
### 3. **Interpreting the Goniometer's Output:**
- **Mono Signals (Centered Sound):**
When the left and right channels are identical, the stereo signal is effectively mono. This is represented on the goniometer by a straight diagonal line running from the bottom left to the top right (45° line). This line means the left and right channels are perfectly in phase and balanced equally, indicating no stereo spread.
- **Stereo Signals (Wide Stereo Image):**
A wide stereo image will produce a more complex, spread-out pattern on the goniometer. If the left and right channels have significant differences in phase, amplitude, or panning, the display will form a wider, more diffuse shape. For example:
- **Sound panned to the left** produces a vertical tilt towards the left side of the display.
- **Sound panned to the right** tilts the shape towards the right.
- **Phase Differences (Out-of-Phase Information):**
When the left and right channels are out of phase (timing differences between channels), the goniometer will display diagonal lines or shapes that deviate from the 45° line. If the phase difference becomes large (for instance, 180° out of phase), the goniometer may show a pattern that shifts towards a horizontal line (from top left to bottom right), indicating an issue that could lead to phase cancellation when summed to mono.
- **Phase Correlation Indicator (Polarity):**
Many goniometers have a correlation meter alongside the display. This meter shows the phase correlation between the left and right channels, ranging from +1 (perfectly in phase) to -1 (completely out of phase). A correlation close to **+1** is ideal for stereo audio, while a correlation approaching **-1** suggests potential phase issues that could cause problems in mono playback.
### 4. **Types of Information Displayed:**
- **Stereo Balance:** By looking at how symmetrical the pattern is, an engineer can determine if the left and right channels are balanced in amplitude.
- **Stereo Width:** The "spread" of the pattern shows how wide or narrow the stereo image is. A narrow pattern means a narrow stereo image (more mono), while a wider pattern indicates a broader stereo field.
- **Phase Issues:** Patterns that are too far off the diagonal axis may indicate phase issues, especially if the goniometer displays shapes leaning horizontally rather than vertically.
### 5. **Practical Use:**
Audio engineers use a goniometer to:
- **Visualize stereo balance** and ensure that both channels are appropriately contributing to the stereo image.
- **Identify phase problems** that could cause poor audio quality when stereo signals are collapsed into mono.
- **Monitor stereo width** and adjust the spread of sound in a mix, ensuring that the stereo image is wide enough to create an immersive experience but not so wide that it causes phase cancellation or balance issues.
### Summary:
A goniometer displays stereo information by graphing the left and right audio signals against each other, creating visual patterns that represent the amplitude and phase relationships of the stereo signal. Engineers can interpret these patterns to monitor the stereo width, balance, and potential phase issues in their audio, ensuring that the sound will work well in both stereo and mono playback systems.