How about DPA protection? Need to protect against Differential Power Analysis that can 'sniff' out secret keys by looking at power differences during crypto functions... Very common in electronic transaction cards...
"Although 4K content is still limited, and the timing for 4K rollout differs from one operator to another, service providers "are watching the timeline carefully" in order to migrate from 1080p to 2160p. For that reason, Marvell made the new chip "pin compatible" with the company's previous chip, ARMADA 1500 Pro, unveiled earlier this year. Marvell's new ARMADA 1500 Pro 4K SoC features 3840 x 2160 p60 10 bit HEVC video decode capabilities."
It's a good thing that the service providers are waiting for that moment when FHD turns to 4K, because if they have the transition way faster, then if the standards change then their chip lineup designs would have to change too. It may be future-ready, but it cannot cope up with changing standards.
Both MIPS and PowerPC had ruled the set top box market for years. But I hear some speculation that both Broadcom (MIPS) and ST (own processor) -- two leaders of this market segment -- may be switching to ARM.
From what I've read over the years, they've been mostly ARM or PowerPC, now pretty much all ARM (I suspect). Marvell makes a lot of these, based on their ARM family which -- ironically -- they bought from Intel (XScale) who in turn had acquired it from DEC (StrongARM).
There may be some MIPS processors in STBs.
Most x86 processors use too much power for a fanless STB.
The real work of an STB nowadays is video decode, which generally needs to be done in a GPU. So the CPU is mostly doing networking and control -- it doesn't need x86 performance so use something cheap and cool.
Update: I once used an IBM PPC 401GF which was originally designed for STBs. Delightful little processor: 32-bit PowerPC in an 80-pin TQFP with multiplexed address/data bus.
I was a little surprised to see an ARM chip being targeted for a high-end STB application. Is that unique to the new generation or am I behind the times? My impression was that previous STB devices tended to use Intel or other architectures. I knew that ARM was making significant advancement into areas like this, but I had not realized that they were mainstream.