Dash Cam Main Stream vs. Substream: Video Bitrate Guide | GreatWill

When a dash cam supports both a main stream and a substream, the two video feeds are designed for different jobs. The main stream normally preserves more detail for local recording and incident review. The substream uses lower resolution, bitrate, and sometimes frame rate to make remote viewing practical over 4G networks.

This distinction matters in fleet deployments. Using the main stream for continuous live viewing increases mobile data usage, upload bandwidth, server load, decoding pressure, and buffering risk. A better setup is usually to record the main stream locally and use the substream for routine remote monitoring. For the broader fleet-management context, see why 4G dash cams are useful for commercial fleets.

This guide explains the difference between dash cam main streams and substreams, why a substream should reduce both resolution and bitrate, how much data it can save, and when a fleet platform should switch back to the main stream.

What is the main stream in a dash cam?

The main stream is normally the higher-quality video feed generated by the dash cam. It usually uses a higher resolution, higher bitrate, and the full recording frame rate configured for the camera.

• Local SD card or onboard storage
• Accident recording and evidence review
• Checking license plates and road signs
• Reviewing driver behavior and collision details
• Downloading the original video after an event

The main stream is not simply a marketing setting called “HD.” Its practical value depends on the complete imaging chain, including the image sensor, lens, exposure control, frame rate, codec, and bitrate.

What is the substream in a dash cam?

The substream is a secondary video feed designed for lower-bandwidth access. It is normally generated with a lower resolution and lower bitrate than the main stream. It is mainly used for live viewing through a mobile app, remote monitoring over 4G, fleet platform previews, multi-camera dashboards, online-status checks, and fast playback in weak network conditions.

A substream is not a replacement for the main recording. It is a separate output optimized for transmission and preview. If the platform displays a low-resolution substream, the device may still be recording the higher-quality main stream locally.

Main stream vs. substream: What is the difference?

• Main stream: higher resolution and bitrate, larger files, better suited to local recording, evidence review, license-plate review, and detailed incident investigation.
• Substream: lower resolution and bitrate, smaller files, lower 4G consumption, and better suited to routine remote preview, multi-camera dashboards, and general scene awareness.

The exact behavior depends on the device firmware and video platform. Some systems call these feeds “main stream,” “substream,” “secondary stream,” or “preview stream.” The names may differ, but one feed prioritizes detail while the other prioritizes efficient transmission.

Why should a substream reduce both resolution and bitrate?

Resolution determines how many pixels must be processed in each frame. For example, 1920 × 1080 contains approximately 2.07 million pixels, 2560 × 1440 contains approximately 3.69 million pixels, and 3840 × 2160 contains approximately 8.29 million pixels.

Bitrate determines how much data is available to represent those pixels every second. If resolution remains high but bitrate is reduced too aggressively, the encoder may produce block-shaped artifacts, smeared moving edges, lost road texture, unstable license-plate characters, visible night noise, and motion blur.

Reducing resolution and bitrate together gives the encoder a more realistic data budget. A well-matched 720p substream can be more useful for remote viewing than a heavily compressed 1080p stream. Resolution alone does not determine evidence quality.

How much data can a substream save?

Data saving percentage = (main stream bitrate − substream bitrate) ÷ main stream bitrate × 100%.

With a 4 Mbps main stream and a 0.5 Mbps substream, the substream reduces theoretical video data by 87.5%. Continuous transmission would represent approximately 1.8 GB per hour for the 4 Mbps stream and 0.225 GB per hour for the 0.5 Mbps stream, before protocol overhead, retransmission, signaling, and other non-video traffic.

For four cameras on one vehicle, four 4 Mbps main streams require approximately 16 Mbps of video bitrate, while four 0.5 Mbps substreams require approximately 2 Mbps. For a fleet of 100 vehicles, the theoretical totals are approximately 1.6 Gbps and 200 Mbps respectively. For platform-level context, see GreatWill’s 4G dash cam solution page.

What happens if the main stream is used for live viewing?

• 4G data consumption rises quickly.
• Weak networks become more sensitive to delay, frozen frames, buffering, and reconnection.
• Multi-camera viewing multiplies uplink and platform bandwidth.
• Phones, browsers, and monitoring computers require more decoding power.

The correct operating rule is simple: use the substream for continuous awareness and use the main stream for detailed investigation, evidence review, and export.

When should a platform use the main stream?

Use the main stream when the operator needs details that a substream cannot reliably preserve, such as reviewing a collision, checking a license plate, reading a road sign, investigating driver behavior, or exporting evidence.

1. Use the substream to confirm the vehicle, camera, and approximate event time.
2. Open the relevant channel and time range.
3. Retrieve the main-stream recording from local storage or the video platform.
4. Review the original file at full resolution.
5. Export the required clip without relying on a compressed preview copy.

How should the two streams be configured?

Configure the main stream around the evidence requirement, storage capacity, sensor, lens, frame rate, bitrate, exposure, and codec. Configure the substream around the remote viewing window and operator task. Low-resolution streams suit status checks; 720p-class streams often suit single-camera previews; 1080p-class substreams require stable 4G and should not be the default for large multi-camera dashboards without capacity testing.

H.265/HEVC can often achieve similar visual quality at a lower bitrate than H.264, but it does not eliminate the need for a substream. Test compatibility across the dash cam, 4G module, platform, mobile app, browser, cloud recording, playback, and exported file. For OEM or wholesale deployments, GreatWill’s dash cam wholesale solutions provide a starting point for project discussions.

How to verify whether the two streams are configured correctly

Test the main stream in daylight, at night, during high-speed movement, rain, strong backlight, and simultaneous multi-camera recording. Test the substream under strong and weak 4G conditions, single-camera and multi-camera viewing, multiple vehicles online, and short network interruptions. Confirm that the platform is actually calling the substream.

For fleet deployment, record the actual bitrate per channel, daily live-view duration, simultaneously viewed channels, monthly SIM data usage, local storage retention, and cloud traffic. The main stream must meet the evidence requirement; the substream must provide usable remote visibility.

Final recommendation

For most commercial dash cam deployments, the main stream should handle local recording and detailed incident review. The substream should handle routine live viewing, multi-camera preview, and low-bandwidth remote access. Use the substream to find and monitor an event; use the main stream to investigate it.

Frequently asked questions

Sources and technical references

• Swann Security Camera App — software and remote viewing reference
• SCW Software & App Downloads — security-camera software and remote access reference
• Nexar: Dash Cam Bitrate vs. Resolution — bitrate and resolution comparison
• BlackboxMyCar: Dash Cam Video Bitrate reference — dash cam technical resources
• Tencent Cloud Developer Community — cloud video and streaming reference