While FFmpeg is streaming, open a second terminal and run vcgencmd measure_temp and vcgencmd get_throttled. The first reports the SoC temperature; the second reports current and since-boot voltage, frequency-cap, throttling, and soft-temperature-limit flags. Read both together: a nonzero throttle value alone does not prove that heat is the cause.
Check temperature and throttle flags during the stream
- Start the FFmpeg stream that normally causes the problem. Use the same resolution, frame rate, filters, and encoder settings you want to diagnose; workload affects heat and CPU demand.
- Open a second terminal and read the SoC temperature:
vcgencmd measure_temp. Raspberry Pi documents this as an instantaneous reading obtained by communicating with the GPU. [Raspberry Pi documentation] - Read throttle status:
vcgencmd get_throttled. Record the hexadecimal output and the time alongside whether the stream was idle, starting, or under sustained load. - Repeat the readings during representative operation. A single sample may miss an intermittent event. Raspberry Pi’s Pi 5 article demonstrates logging temperature, Arm clock, and throttling state over time. [Raspberry Pi Pi 5 thermal testing]
For an alternate temperature reading, Raspberry Pi documents cat /sys/class/thermal/thermal_zone0/temp; divide the returned integer by 1000 to get degrees Celsius. Raspberry Pi cautions that Linux-based temperature readings can be inaccurate on its SoCs, and describes vcgencmd measure_temp as an accurate instantaneous reading. [Raspberry Pi documentation]
Decode the hexadecimal throttle value
get_throttled is a bitmask: each set bit identifies a condition. Raspberry Pi OS documents these meanings. [Raspberry Pi documentation]
| Bit | Hex | Meaning |
|---|---|---|
| 0 | 0x1 |
Undervoltage detected now |
| 1 | 0x2 |
Arm frequency capped now |
| 2 | 0x4 |
Currently throttled |
| 3 | 0x8 |
Soft temperature limit active |
| 16 | 0x10000 |
Undervoltage has occurred since boot |
| 17 | 0x20000 |
Arm frequency capping has occurred since boot |
| 18 | 0x40000 |
Throttling has occurred since boot |
| 19 | 0x80000 |
Soft temperature limit has occurred since boot |
Bits 0–3 describe conditions active now; bits 16–19 report that the corresponding event has happened at least once since boot. Historical flags can remain relevant even when the current condition is gone. Interpret individual bits rather than treating every nonzero result as overheating.
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Tell thermal throttling from a power problem
Raspberry Pi documentation says Arm cores are progressively throttled between 80°C and 85°C; above 85°C, the Arm cores and GPU are throttled. It separately gives Raspberry Pi 3 Model B+ a default soft temperature limit of 60°C; that model-specific setting should not be applied to every Pi. These are documented thresholds, not a universal target for every board or workload. [Raspberry Pi documentation]
Undervoltage can also cause throttling. Raspberry Pi says the supply should remain above 4.8 V for reliable performance and that a drop below 4.63 V (±5%) causes the Arm cores and GPU to throttle. For Raspberry Pi 5, vcgencmd pmic_read_adc EXT5V_V reads the PMIC’s external 5 V input. Use power guidance appropriate to the exact board. [Raspberry Pi documentation]
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- If current thermal flags coincide with readings in the documented thermal region during the stream, heat is a plausible cause.
- If voltage-related bits are set, or throttling appears without temperatures approaching the documented thermal region, investigate the supply and the specific bit meanings before changing cooling.
- If only since-boot bits are set, the event may have occurred before the present stream. Compare a fresh observation over time with the current-state bits.
Why FFmpeg settings and Pi model matter
Sustained video processing can keep a Pi under load rather than giving it short idle periods to cool. Raspberry Pi’s cooling paper describes video processing as a sustained, compute-intensive workload. Its H.264 encoding paper compares Pi 5 software libx264 configurations with the Pi 4 hardware encoder h264_v4l2m2m; it characterizes a low-latency software configuration as intended for real-time streaming and a higher-quality software configuration as using more CPU and latency. Those examples explain why the board and encoding path matter, but do not predict the performance of your FFmpeg build, filters, resolution, or frame rate. [Raspberry Pi cooling paper] [Raspberry Pi H.264 encoding paper]
Diagnose the actual configuration that misbehaves. Compare its logged readings with idle or lighter-load readings, and note whether thermal or voltage flags appear as the stream runs. A result from another Pi model or encoder path is not a substitute for measurements on your setup.
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What to change if measurements point to heat
- Check the board’s airflow, ambient temperature, and sustained workload.
- Check whether the case is airtight or obstructs airflow, and confirm any cooling accessory is compatible with the exact board and case.
- If the normal workload still shows thermal throttling, consider a suitable heatsink or small fan. Raspberry Pi says cooling may help in some circumstances, while most use cases do not need extra cooling; airflow improves heatsink cooling. [Raspberry Pi computer documentation] [Raspberry Pi cooling paper]
- Repeat the same monitoring under the same stream configuration to see whether thermal flags still occur. Do not assume an accessory will improve an unmeasured setup.
Common readings and what to do next
| What you see | Likely interpretation | Next check |
|---|---|---|
| Temperature rises under load; current throttling or soft-limit bit is set | Thermal throttling is plausible | Review airflow, ambient temperature, enclosure, and board-compatible cooling. |
| Undervoltage bit is set, with or without a thermal flag | Supply voltage may be contributing | Check the power supply and board-specific power guidance; on Pi 5, consider the PMIC voltage reading. |
| Only since-boot bits are set | An event occurred earlier; it is not necessarily happening during this stream | Record current bits and temperature over time during the stream. |
| Throttle output is nonzero but no current condition is apparent | The value may reflect a historical event or a nonthermal cause | Decode each set bit and compare with temperature and supply readings. |
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