Left total/Right total – a two-channel audio format that contains matrix-encoded surround sound information, allowing compatible decoders (Dolby Pro Logic) to extract center and surround channels from the stereo signal while remaining compatible with standard stereo playback.
What is LtRt?
LtRt (Left total/Right total) is a matrix-encoded two-channel format that contains surround information. Compatible decoders like Dolby Pro Logic can extract center and surround channels from the stereo signal.
Basic Principle
| Aspect | Description |
|---|---|
| Meaning | Left total/Right total |
| Channels | 2 (with encoded data) |
| Surround Info | Matrix-encoded |
| Decoding | Pro Logic compatible |
LtRt vs. LoRo
| Aspect | LtRt | LoRo |
|---|---|---|
| Surround Info | Yes (encoded) | No |
| Decoding | Possible | Not necessary |
| Complexity | Higher | Simple |
| Phase Relationship | Critical | Less critical |
Matrix Encoding Principle
| Channel | Encoding |
|---|---|
| Center | L = R (in phase) |
| Surround | L = -R (out of phase) |
| Left | L only |
| Right | R only |
Encoding Formula
| Source | Lt | Rt |
|---|---|---|
| L | 1.0 | 0 |
| R | 0 | 1.0 |
| C | 0.707 | 0.707 |
| S (Mono) | 0.707 | -0.707 |
Decoding (Pro Logic)
| Step | Description |
|---|---|
| Sum | Lt + Rt = Center |
| Difference | Lt - Rt = Surround |
| Steering | Directional recognition |
| Output | L, C, R, S |
Pro Logic Versions
| Version | Description |
|---|---|
| Pro Logic | Original, mono surround |
| Pro Logic II | Stereo surround |
| Pro Logic IIx | 6.1/7.1 expansion |
| Pro Logic IIz | Height channels |
Application Areas
| Area | Usage |
|---|---|
| TV Broadcast | Standard delivery |
| DVD | Stereo track |
| Streaming | Surround compatible |
| Archiving | Compact format |
Advantages
| Advantage | Description |
|---|---|
| Backward Compatible | Works as stereo |
| Surround Potential | With decoder |
| Compact | Only 2 channels |
| Established | Proven technology |
Disadvantages
| Disadvantage | Description |
|---|---|
| Channel Separation | Limited (~30dB) |
| Phase-Critical | Can create artifacts |
| Mono Surround | In basic Pro Logic |
| Compromise | Not like discrete 5.1 |
Creation
| Method | Description |
|---|---|
| DAW Encoder | Plugin |
| Hardware | Dolby encoder |
| Console | Downmix matrix |
| Software | Dedicated tools |
Software Tools
| Tool | Function |
|---|---|
| Dolby Encoding | Professional |
| Pro Tools | Downmix options |
| Nuendo | Encoder integrated |
| Plugin Solutions | Various |
Quality Control
| Check | Method |
|---|---|
| Decode Test | Check Pro Logic |
| Phase Check | Correlation meter |
| Mono Test | Cancellations? |
| Dialogue | Center extraction |
Phase Problems
| Problem | Cause |
|---|---|
| Dialogue disappears | Phase error |
| Surround bleeding | Incorrect matrix |
| Mono collapse | Out-of-phase material |
| Pumping | Steering artifacts |
Delivery Requirements
| Platform | Specification |
|---|---|
| Netflix | LtRt required |
| Broadcast | Standard |
| Streaming | Often needed |
| Home Video | As an option |
Technical Specifications
| Parameter | Typical Value |
|---|---|
| Sample Rate | 48kHz |
| Bit Depth | 24-bit |
| Loudness | -24 LKFS |
| True Peak | -2 dBTP |
Cinema History
| Year | Development |
|---|---|
| 1976 | Dolby Stereo introduced |
| 1987 | Pro Logic |
| 2000 | Pro Logic II |
| Today | Still relevant |
Workflow in Post
| Phase | Activity |
|---|---|
| 5.1 Mix | Surround version |
| LtRt Encode | Matrix downmix |
| QC | Decode verification |
| Delivery | Along with LoRo |
Comparison to Discrete 5.1
| Aspect | LtRt | Discrete 5.1 |
|---|---|---|
| Channels | 2 | 6 |
| Separation | ~30dB | Complete |
| File Size | Small | Large |
| Quality | Good | Best |
Best Practices
| Practice | Reason |
|---|---|
| Check decode | QC essential |
| Observe phase | Avoid artifacts |
| Compare with LoRo | Consistency |
| Documentation | For acceptance |
Today
LtRt remains an important component of professional audio deliveries, even though discrete surround formats dominate. The ability to transport surround information in a 2-channel format makes it valuable for backward-compatible playback. The correct creation and quality control of LtRt are part of the standard repertoire of any audio post-production.