How to select the Best Quality HDMI Active Optical Cable (AOC)

Recently, there are various prices for fiber optic HDMI cables on the market, take a 10 metre cable for example, the unit price range is from $10.00/pc up to $100.00/pc or even more. Some manufacturers say that the HDMI fiber optic cable is just a high-frequency cable that has no technical content to compare, and it can be guaranteed for life. This made me thoroughly understand the phrase "Ignorance is bliss"! Smartavlink has been working in AOC technology for more than 6 years. Here we take the opportunity to introduce and explain why there are so many different AOC on the market. For the end-buyers, you will probably know how to select a good quality AOC after reading this article.

Smartavlink started AOC development with TOSA & ROSA from the beginning with HDMI V1.3 and has been modularized by today's COB (Chip On Board) process. During these years we had experienced countless failures and endless testing. Some of the main factors, for example “Compatibility Issues”, “I²C communication”, “Reliability & Stability”, have been continually improved on the production line to ensure our AOC is more compatible with a wider range of devices on the market. In my personal understanding of HDMI AOC, there are four aspects that need attention:

• AOC product generation background
• AOC module design
• AOC cable process requirements
• AOC finished product quality control

This picture shows the first generation HDMI1.3 AOC developed by Smartavlink in 2013, using the design ideas of TOSA & ROSA.

AOC is the acronym for “Active Optical Cable”. With this design, the idea is to convert the high-speed digital signal, such as HDMI/DP/USB, into optical signal, as traditional copper cable has limitations on transmission distance due to its natural physical properties. HDMI/DP/USB AOC all take fiber as special medium, in theory, the bandwidth can be increased without limitation and there is no signal attenuation over longer distances. The market always shows great potential need for long distance transmission applications. It creates greater possibilities of applications for HDMI/DP/USB etc.

Nowadays, the application of high-definition video is becoming more and more popular, from personal mobile phones to commercial Screens, industrial and medical, etc. More and more scenarios are provided by AOC application requirements and AOC will offer even more applications with the arrival of 5G. Based on this background, the Smartavlink team has been consistently engaged in the development, design and manufacturing of HDMI/DP USB fiber optic cable etc. since 2013 but no copper cable. It’s the only product we’ve being working on for 6 years. I believe that as the market division of labor becomes more and more detailed, focus and professional will allow us to live longer and run further.

Firstly, let's talk about the module. The module is mainly composed of the main chip + laser. Some modules are composed of main chip + laser + DDC buffer chip. At present, the most common in the market is the German brand chip (Silicon line) + Phillips matrix laser (VCSEL), and some modules have also added Texas Instruments (TI) DDC Buffer Chip which is used for making longer cable assemblies.

The main features of Silicon's chip are stable performance and low power consumption which had early development and entry into the market - it is the mainstream driver chip right now. Their power consumption of TX is generally around 200mW and RX only about 160mW therefore almost all playback and display devices are compatible.

So on the positive it has extremely good efficiency for low power consumption, but on the downside, it turns out its light eye and the electric eye are not large enough to you end up with too many error codes in the finished stream. When used on high-end devices such as Apple Box and Sony TV combination, there will be screen flashes and noise for individual products. So, Smartavlink developed a fully automatic error code test program for this problem to ensure that each cable can meet the application needs of top professional customers thereby reducing the risk factor of splash screen and noise (see below pictures).

Some module manufacturers chose other chips, since these chips are generally used in optical communication/Data centres, the photoelectric eye effect of these chips is better than Silicon-line, but the biggest problem is greater power consumption. Their TX power consumption is good, generally around 200mW, but RX power consumption often exceeds 300mW. Currently all chip modules need to get power from the display side, which may blacken the display device and reduces the chip's compatibility. Also, some of the devices may be unable to supply such a large amount of power, therefore it will need an external power supply, this makes it inconvenient for the end user using the product.

At present, there are many module manufacturers in this market, mainly from the United States, South Korea, and mainland China, including Taiwan, but overall, there is not a 100% perfect module so far, because HDMI AOC is a non-standard product on the market. All chip and module manufacturers are still working hard on developing this!

Besides the main chip, the laser is also very important. Most modules use Phillips matrix laser (VCSEL), and some modules use II VI whereas some of the higher end users use Finisar laser. Many users know the Silicon-Line driver chip but don't quite understand what VCSEL laser is or, its importance. The figure below shows the working principle of VCSEL used on AOC:

Because all AOCs need covert its electric signal to optical signal first, they then pass optical signal from the Source Module to the Display Module. Then the Display module must convert its optical signal into an electrical digital signal such as HDMI/DP/USB. This will be the complete link for all signal transfer, see the following working principle:

Due to the limitation of the design and manufacturing process, the lasers from different manufacturers have different intensity of optical power and the life of the laser is also inconsistent This will directly lead to the failure of the HDMI AOC cable after being introduced to the market. It may produce faults like Splash screens, black screens, and even direct failures after one or two months. The four-way lasers may stop one at a time altogether and it is a hidden danger. 

In response to this problem, the Smartavlink technical team has developed a unique set of equipment to monitor the optical power consumption of 4-way lasers to ensure that all products have reliable optical paths and reduce the degradation caused by the VCSEL laser. The following picture shows Smartavlink’s privately designed equipment for testing 4x fiber channel signal loss. 

After we tested the optical signal loss, Smartavlink then established a QR code traceability system so that each product has a unique ID card so then each unit can be traced back by more than 10 years since production to ensure customers become more confident choosing Smartavlink's products, and distinguishing these from alternative products and perhaps of lower quality. We believe that it is not just a cable, it is an electronic & fiber optical device, it has more of a job to do than just a cable.

Now that we have explained about the laser, let's talk about the communication protocol chip of Texas Instruments. Firstly, it is important to know that the DDC low-frequency signal is used to implement the handshake protocol. The HDMI Association stipulates that DDC specification of the HDMI product is less than 700pf, but for AOCs, the theoretical value of this cable assembly is 4000pf/100m. In principle, the lower the capacitive reactance of the low-frequency signal cable, the greater the transmission distance can be and signals will also be more stable.

The existing modules have some components to reduce the capacitive reactance and the skilled cable- producing manufacturers can reduce the capacitive reactance to 2600pf/100 metres by changing the cable-producing process, but when making a longer cable, the DDC capacity resistance value will still exceed 700pf, so when we produce a cable longer than 60 metres we still need to use a chip module with a “TI” communication protocol. This is to ensure that the finished products of greater lengths have no problem in terms of protocol communication. Smartavlink’s technicians works from practical experience, started from the fundamental problem and specially targeting the DDC signal to make sure I²C signal can successfully complete the handshake, greatly improving the compatibility and reducing the end users concerns about compatibility. The figure above shows the waveform of the cable as Smartavlink’s technician measures the DDC.

Besides chip module, let’s talk about the AOC wire. At present, the quality of AOC wire is very important, far more than standard HDMI copper wire. Take standard copper wire to make an analogy, some manufacturers use the 30AWG wire on 3 metres of finished products that can pass EMI, bandwidth 18G, some manufacturers draw out the bandwidth to only 13.5G and some manufacturers draw out the bandwidth only 10.2G or even just 5G. Fortunately, the HDMI Association has determined that the copper wire has detailed specifications therefore you know the quality of the cable by comparing it.

However, current AOC cable is troublesome as no standard specification to follow. At present, AOC cables generally go for 4 fibers +7 copper wire structure. 4 fibers are used to transmit data, the 7 copper wires for communication protocols: one for power supply, one for CEC, two for audio return (ARC), and one for DDC signals (two cores for foam plus one ground, the wire is shielded with aluminum foil). It is precisely because there is no standard to rely on, the wire materials used in the market vary widely, and the wire performance can make a big difference. They are respectively OM1.5, OM2, OM2+, OM3, OM4, and the core protection is PVC and Teflon. Filling materials, such as Kevlar, Nylon, PP, and cotton. The outer jacket is made of PVC, CL2PVC, TPE, CMP, etc. So many different material options make the cost of AOC wire vary greatly and the unit price span range from 1.2 RMB/meter to 15 RMB/meter. The finished wire has a great performance differential also.

Here I will talk about the wire processing of the ARC function and the capacity reactance of the DDC wire. Now, more and more users are paying attention to the ARC function of the fiber optic cable. Our conventional wire can only support ARC up to 15 metres and many users require support up to 30 meters or even 50 meters, So we have to do something special with this set of core wires and the conductor specification is changed by foaming and aluminum foil shielding. The processing is good and the audio sound of the finished cable under 75 meters can be returned without any problem. When I talked about the module chip in the previous section, I mentioned the importance of the DDC wire capacitive reactance value. After hundreds of tests, we extracted the 2600pf/100m wire (the theoretical value is 4000pf/100m). Pumping AOC cables still requires a lot of technical skills. If the wire is not well drawn, the function of the finished wire will be missing, the compatibility will be poor also.

Finally, let’s talk about the finished product assembly of the AOC cables. The FA45, LENS, and the ceramic film are commonly used in the market as the medium for optical signal transmission. These programs have their own advantages and disadvantages. Many traditional HDMI copper cable factories are too optimistic about the market, blindly buying modules without understanding them, just simply connect it and it’s considered to be a qualified HDMI AOC, always aiming for lower price but then sacrifice item functionality. It is destroying the confidence of customers buying AOC products. Since optical fiber HDMI used the photoelectric conversion chips, lasers optical devices etc., the problem generally takes 3 to 6 months or more to be founded out.

A quality-conscientious manufacturer will partially prepare the fiber core with the proper amount of UV glue after cutting the fiber and use a BER tester to detect the bit error rate and bandwidth during the process. You also need to use MURIDEO to check if the data will be lost. A cable can be tested on a TV without issue and can be passed off as a QC controlled cable even without the tests here have not been performed. Unfortunately, too many error codes usually shows up after the TV has been playing for a few hours or even a dozen hours which will need returning therefore damaging the reputation of the product.

In summary, we have been continuously producing and improving the quality of optical fiber HDMI for 6 years. Found and solved numerous technical problems, we also feel honored that there are dozens of brands that choose the fiber HDMI, DP and USB cable made by us. In the past six years, Smartavlink has developed from the original fiber optic HDMI to today's complete and rich photoelectric conversion series product line as below:

• HDMI2.0 Type A AOC HDMI2.0 Type D AOC
• HDMI2.0 Armored Type A AOC
• HDMI2.0 Armored Type D AOC
• USB3.0 Male to Male AOC
• USB3.0 Male to Female AOC
• Type C GEN2 10G AOC
• USB3.0 Male to TYPE C AOC
• DP1.4 to HDMI AOC
• MINI DP1.4 to HDMI APC
• HDMI2.1 Type A AOC

Except for above standard products, Smartavlink also helps customers design and develop special AOC solutions for special applications, including applications from commercial to industrial, machine vision to medical etc.

After CES show in 2019, major TV manufacturers demonstrated strong 8K applications and manufacturing capabilities and with the emergence of the commercial application of 5G, it accelerated the application requirements of 8K and higher bandwidth. The following picture shows the forecast for 8K TV. We can clearly see the huge application prospects of 8K and the growth of the market, so it also brings a lot of chance and challenge to AOC cable.

Smartavlink has been working hard at HDMI AOC field, listening to the voice of customers and quietly improving products which has won the recognition of numerous professional users. Because of such persistence, we show that we are professional! Thanks to the support from our suppliers and partners all the time during these 6 years has Let Smartavlink, in such a chaotic market, focus on quality and continuous development. Let's work together and walk further in the future.

Smartavlink team has confidence and capability to use their technical advantages and R&D capability to help customers solve more application problems with the concept of photoelectric conversion.

ITM Components are the main supplier of SmartAVlink AOCs in the UK.