Maximum number of channels with one BUS of Thunderbolt

Performance Considerations and Testing Recommendations

Performance tests have been conducted on specific computer systems and reflect typical results under those conditions. Actual performance may vary depending on your setup, including the computer model, macOS version, and driver versions. We strongly recommend running your own tests using our free demo versions to evaluate how the system performs in your specific workflow.

Thunderbolt Version Considerations

The maximum number of video channels that can be transmitted over a Thunderbolt bus depends on the available PCIe bandwidth—which varies between Thunderbolt versions. It’s important to note that the total Thunderbolt bandwidth is shared between PCIe, DisplayPort, and other data streams. The PCIe bandwidth specifically refers to the portion allocated for data transfer to and from PCIe devices, which is what matters for video ingest and output. The table below compares the typical PCIe configuration and theoretical bandwidth across Thunderbolt versions.

TB Version	PCIe Gen	PCIe Lanes	Theoretical Max Bandwidth(PCIe only, per direction)	Total Bandwidth(incl. DisplayPort, per direction)
1	PCIe Gen 2	x4	2 GB/s	10 Gbps (1.25 GB/s)
2	PCIe Gen 2	x4	2 GB/s	20 Gbps (2.5 GB/s) (aggregated dual-channel)
3	PCIe Gen 3	x4	4 GB/s	40 Gbps (5 GB/s)
4	PCIe Gen 3	x4	4 GB/s	40 Gbps (5 GB/s) (same as TB3 but stricter requirements)
5	PCIe Gen 4	x4	8 GB/s	80 Gbps (10 GB/s), up to 120 Gbps with Bandwidth Boost

Notes:

• PCIe bandwidth figures above represent theoretical maximums for PCIe tunneling (not including overhead).
• Total bandwidth includes other traffic (e.g., DisplayPort, USB), so actual PCIe throughput may be lower in mixed-use scenarios.
• Thunderbolt 5 introduces a dynamic Bandwidth Boost mode that can temporarily allocate up to 120 Gbps (3:1 asymmetric) for heavy display or data use.

About Bandwidth Direction: Can You Send and Receive Simultaneously?

One common question is whether Thunderbolt bandwidth is bidirectional, and how that impacts workflows involving both input and output—such as receiving and sending multiple video channels at the same time.

Thunderbolt provides full-duplex (bidirectional) bandwidth. This means the available bandwidth is independent in each direction—you get the full theoretical bandwidth for sending and the full bandwidth for receiving, simultaneously. For example, with Thunderbolt 3, you have up to 40 Gbps in each direction (with roughly 4 GB/s available for PCIe in each direction).

However, real-world performance can still vary due to:

• Thunderbolt controller limitations
• Expansion chassis architecture
• macOS resource allocation
• Other devices sharing the same Thunderbolt bus
• Video card capability: the card used in the chassis will also have bandwidth limits

So in theory, if two 4K60p channels (at the upper limit for Thunderbolt 3) are being received, you should still have enough bandwidth in the opposite direction to send two more 4K60p channels—provided the hardware and drivers fully support that load.

That said, we always recommend running real-world tests with your specific setup to validate performance.

Choosing the expansion chassis

Not all Thunderbolt expansion chassis deliver the same performance. Consider the following factors:

• Thunderbolt version: The version of Thunderbolt in use can also impact overall bandwidth. Newer versions, such as Thunderbolt 5, offer significantly higher bandwidth—up to twice that of earlier generations—which can benefit demanding workflows. Make sure that both the chassis and the computer support the Thunderbolt version.
• PCIe generation: For optimal speed, use expansion chassis with PCIe Gen 3 slots. Chassis with PCIe Gen 2 slots will offer significantly lower performance.
• Electrical lane configuration: In Sonnet expansion chassis for example, some slots are x8 electrical, while others are x4. For best results, use the x8 slots, particularly when working with PCIe gen2 cards.
• Thunderbolt ports: Most expansion chassis include two Thunderbolt connectors, but in most case both cannot be connected to the same computer. The second port is for daisy chaining. If you need to connect both ports to your Mac, only Sonnet models with “DV” in the name (e.g., Echo II DV Module) support this, providing separate Thunderbolt buses for each card.

Thunderbolt 3 - Benchmark results table

What are these numbers about?

The tables below show how many video channels can be transmitted over a single Thunderbolt bus—whether for sending or receiving frames. This relates to data throughput, not to the actual performance of encoding or decoding those frames, which depends on other factors such as CPU/GPU capabilities. If you have questions about your specific setup, feel free to contact us.s.

When using a Thunderbolt expansion chassis with PCIe gen 3 slots

The following tests were conducted by Softron in June 2024 using a Mac mini M2 Pro or a Mac Studio M1 Ultra, running macOS 14.4, with Decklink Quad 2 and Decklink 8K Pro, using drivers 14.0.1, in a Sonnet Echo Express SE III expansion chassis. The cards were placed in the 8 x PCIe gen 3 electric slots.

Dimensions	Pixel format	FPS	Max number of channels
HD (1920 x 1080)	8bit	p25 / i50	16
		p29.97 / i59.94	14
		p50	10
		p59.94	8
	10bit	p25 / i50	14
		p29.97 / i59.94	12
		p50	8
		p59.94	7
4K (3840 x 2160)	8bit	p25	5
		p29.97	5
		p50	3
		p59.94	2
	10bit	p25	4
		p29.97	3
		p50	2
		p59.94	1
8K (7680 x 4320)	8bit	p25	1
		p29.97	1
		p50	0
		p59.94	0
	10bit	p25	1
		p29.97	1
		p50	0
		p59.94	0

When using a Thunderbolt expansion chassis with PCIe gen 2 slots

Some older expansion chassis still use PCIe gen 2 slots (such as the xMac mini Server), and thus the speed will be slower. The following tests were conducted by Softron in June 2024 using a Mac mini M2 Pro or a Mac Studio M1 Ultra, running macOS 14.4, with Decklink Quad 2 and Decklink 8K Pro, using drivers 14.0.1, in a x Mac mini Server expansion chassis. The cards were placed in the PCIe gen 2 slots.

Dimensions	Pixel format	FPS	Max number of channels
HD (1920 x 1080)	8bit	p25 / i50	15
		p29.97 / i59.94	12
		p50	5
		p59.94	5
	10bit	p25 / i50	11
		p29.97 / i59.94	9
		p50	5
		p59.94	4
4K (3840 x 2160)	8bit	p25	3
		p29.97	3
		p50	1
		p59.94	1
	10bit	p25	2
		p29.97	2
		p50	1
		p59.94	1
8K (7680 x 4320)	8bit	p25	0
		p29.97	0
		p50	0
		p59.94	0
	10bit	p25	0
		p29.97	0
		p50	0
		p59.94	0

About Thunderbolt 5

Recent Macs now support Thunderbolt 5, which—at least in theory—offers double the bandwidth of Thunderbolt 3. This means it should be possible to send and receive up to twice as many video channels over a single Thunderbolt bus. However, we have not yet been able to verify this in practice. We will update this article as soon as we’ve had the opportunity to test with a Thunderbolt 5 expansion chassis.