精密實驗室系列：視訊介面

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Hello and welcome to TI Precision Labs. In this series, we're going to discuss Serial Digital Interface, or SDI. SDI is used for video transmission and television stations, high end video capture, and storage applications.

In this first session, we're going to discuss elements involved in the SDI signal path and the effects of its transmission media, 75 ohm coax, and its characteristics. Then we will discuss how SDI data coding and framing helps improve the performance of SDI components. In future sessions, we will discuss the parametric table for each specific SDI component in more detail.

In an SDI environment, a cable driver driven by a signal source like a camera 100-ohm interface delivers the best quality signals to the 75-ohm coax transmission media with minimal intrinsic jitter. After the signal goes through 75-ohm coax media, the signal information is indistinguishable. This is because the high frequency components of the signal get heavily attenuated.

A coax adaptive cable equalizer on the receiver side restores the original high frequency content and the signal. Next, a clock data recovery phase lock loop recovers the incoming signal clock, and this is used to retime or clock the data. CDR loop filter bandwidth attenuates high frequency noise while letting low frequency noise or jitter pass through. Note that equalizer rate detector filters are used by the CDR to change loop filter bandwidth based on the data rate.

Now let's look at coax cable characteristics in detail to see how it causes signal eye closure. The main media for SDI video transmission is 75-ohm coax. This cable has external insulation with a metal mesh shielding and polyethylene dielectric material. The dielectric constant for this material is about 2.4, but some manufacturers add air into the dielectric to make it bendable, which can modify the dielectric constant to about 1.42.

As noted in the equations below, in addition to the dielectric constant, diameter of the inner conductor and dielectric material have to be kept constant in order to keep its characteristic impedance at a constant 75 ohms. Additionally, capacitance and inductions have to be kept as low as possible to minimize the low pass filtering effect.

SDI single-ended 75-ohm transmission media has a low pass filtering effect, which insinuates high frequency content of the data pattern. The insertion loss versus frequency graph in the case that signal attenuation will increase with data rate. This graph shows two very important points. One, a data rate versus cable reach limitation. For example, using a 600-meter cable, your fundamental frequency has to be under 180 megahertz, meaning the data rate has to be 360 megabits per second or less.

Two, equalize or gain dynamic range. For this example equalizer, the gain should be at least 50 dB. Note that we need to at least have a 10 to 15 dB signal to noise ratio in order to recover the signal. For example, given a 720-millivolt cable driver launch amplitude, the equalizer noise floor has to be in the order of 0.7 millivolts. This requirement demands exceptional analog design and a low noise process technology.

On the other hand, a 300-meter coax cable at 180 megahertz has an insertion loss of about 25 dB. In this case, our 720-millivolt signal would be attenuated to about 40 millivolts. And thus it is more easily recoverable. By the same token, an 80-meter coax cable would have a loss of about 50 dB at 6 gigahertz. This means 80-meter cable reach is achievable at 12 megabits per second, provided our equalizer has a gain of 50 dB with a 10 to 15 dB signal-to-noise ratio.

Let's look deeper into what contributes to insertion loss and low pass filtering effects of the SDI 75-ohm transmission media. There are three types of losses for SDI 75-ohm coax media. There is resistive loss, which is typically specified as dB loss per unit length. This loss predominantly affects low frequency content of the video signal.

Next, we have the skin effect, which specifies the depth of electric field penetration. It is proportional to 1 over the square root of the frequency. Finally, we have dielectric loss, which is proportional to the signal frequency and loss tangent of the media. As we just discussed, coax cable attenuates high frequency content. Therefore, dielectric loss is what we need to be most concerned about.

There is a crossover frequency where skin and dielectric losses are the same. For a frequency less than crossover, the skin effect is more dominant by 1 over the square root of f. Of frequency above fe, the dielectric loss is more dominant. Dielectric loss has a higher slope than skin effect over frequency. Note the lower cable capacitance in conductance, the higher the crossover frequency. The higher crossover frequency generally indicates a higher quality cable.

While an adaptive equalizer device uses gain to restore the signal, data coding or data framing techniques are used to spread the data pattern frequency content. The Society of Motion Picture and Television Engineers, or SMPTE, provides detailed specifications for each step of the video data processing. This is done to guarantee interoperability and quality between different video equipment.

For example, the SMPTE 274 specification calls for scaling equations. This is done to enable the same baseline video and audio level. Additionally, a 3-dB low pass filter cutoff frequency is specified to eliminate aliasing effects. SMPTE also specifies ADC sampling rates for digitizing RGB signals.

Last but not least, SMPTE 274 specifies interleaving and scrambling of the signal. This is done to spread the signal across the whole bandwidth to enable longer cable reach. Another effect of the scrambler is to provide a DC-balanced data pattern.

In this scope shot, we are showing differences between these two patterns. The DC-balanced data midpoint is constant, while the non-DC-balanced data shows a shift in DC value. This shift is called DC wander. A non-DC-balanced data pattern reduces cable equalizer performance. Additionally, the CDR jitter would increase in the presence of this pattern.

Even though SMPTE calls for a scrambler to enable DC-balanced data, research by Takeo Iguchi later showed there could be cases where the SMPTE-specified scrambler could produce a non-DC-balanced data pattern. This non-DC-balanced data pattern is called a pathological video pattern. An equalizer pathological pattern has 19 zeros followed by a single one, or vice versa. This pattern can stress coax equalizers. There is also another pattern for stressing SDI retires called PLL pathological. This pattern has 20 zeros followed by 20 ones, or vice versa.

To refresh your mind on what we just discussed, let's go over a short quiz. Check all correct statements. A, SDI cable driver converts 75-ohm coax with minimal additive jitter. B, adaptive collects equalizer uses the same high frequency Boost Follower all data rates. C, SDI reclocker attenuates high frequency jitter, while low frequency jitter goes through. D, SDI reclocker attenuates low frequency jitter while high frequency jitter goes through.

The correct answers are A and C. Let's move on to the next question. Check all correct statements. A, for NRZI pattern, there is a change in data when there is a one. B, NRZI makes SDI pattern polarity insensitive. C, DC-balanced data means there is the same number of zeros and ones in a pattern. D, pathological data pattern is not DC-balanced.

The correct answers are A, B, C, and D. Finally, at high frequency, which loss is more dominant? Is it dielectric, skin effect, or DC loss due to media resistance?

The answer is A. Dielectric loss is proportional to 1 over f versus the skin effect, which is proportional to 1 over the square root of f. So dielectric is more dominant.

In closing, I hope you enjoyed What Is SDI and Its Media Characteristics. In future sessions we will go into more details and discuss specific parameters governed by the SMPTE. Please visit the TIE2E community at TI.com for questions and additional information.