Personal Video Recorder

The Requirement

TiVo revolutionised video-recording. The video-tapes & the error-prone spinning video-head were replaced by a hard-disk, & the increasingly wide TV-schedule was available as dial-up (this is before broadband) service; they had invented the PVR. Regrettably, TiVo's UK-debut in 2000 with the Thomson PVR10UK, failed after an advertising campaign which focussed solely on the ability to pause live TV, & the ≈2007 migration from terrestrial analogue broadcasting to DVB was the final nail in the coffin. Though over a decade has passed since TiVo's fall (except as offered by Virgin Media), other PVRs haven't reproduced the user-experience. In my experience, they're typically:

The Solution

Software

There are a variety of open-source s/w-projects based on GNU/Linux & kodi (formerly XBMC), which provide not only PVR-functionality but can double as a media-player:

Hardware

In 2012 the Raspberry Pi became available. The model 3B provides:

  • a 4-core processor,
  • USB-2.0 ports,
  • Ethernet & WiFi,
  • hardware-accelerated h.264-decoding.

This is sufficiently powerful to run kodi, & is quite inexpensive. Many people have pioneered such solutions, but I've detailed my specific solution & conclusions.

PVR PVR (oblique)
Component Type # Size
Case
Acrylic sheet Transparent 1 A3 × 3 mm
A4 × 5 mm
Tube Aluminium 4 (72 × 12 × 1) mm
(75 × 12 × 1) mm
Foot Rubber 15 mm
Threaded rod Stainless steel M3 × 30 cm
Nut 8 M3
Washer Penny 20 M3 × 12 mm
8 M3 × 9 mm
Computer
Raspberry Pi Model 3B 1
µSD-card Class-10 8 GB
WiFi-adapter LB-link (Realtek RTL8188CUS) 802.11n
Real-time Clock Battery-backed, DS1307
Standoff Male-female 4 M2.5
Bolt
Nut
Cable HDMI 1 Standard
RF
Acrylic bar Square-section 1 4 cm × 5 mm2
Solvent Tensol 12
Cable-tie Black 20 cm × 2.5 mm
Tuner USB 2.0 DVB-T2 2
RF-adapter Male-male
Cable Male-female USB Type-A, 25 cm
RF-splitter 2-way inductive 1
Bolt Hex socket 2 M3 × 8 mm
Nut M3
RF-adapter Female-female 1
Storage
Hard-disk drive SATA 2 2.5", 500 GB
Adapter SATA 3 to USB 3.0 Standard, Type-A
Bolt Hex socket 8 M3 × 8 mm
Grommet Rubber 10 mm × 3 mm
Flash-drive USB 1 Standard, Type-A
USB-hub
USB-hub USB 3.0 1 4-port, 15 W
Cable-tie Black 2 20 cm × 2.5 mm
Status-LEDs
Strip-board 1 64 mm × 25 mm
Operational amplifier LM324 Quad
Standoff Male-female 2 M2.5
Bolt
Nut
LED Red 4 5 mm
Potentiometer Multi-turn 100 
Resistor 0.25 W 47 Ω
Jump-wire 6 10 cm
Power-supply Switch-mode, with UPS-monitor 1 60 W, 230±10 V AC to 12 V DC
DC-DC Buck-converter
  • LCD display.
  • Switch-mode.
  • Adjustable voltage & current-limit.
  • 15 A fuse.
  • Supplied with 4 M3 standoffs.
10 A
Battery VRLA 12 V, 2.1 Ah
Barrel-connector Right-angle 2 A, 1.35 mm × 3.5 mm
µUSB lead Power only 35 cm, 18 AWG
Mains-cable 3-core, PVC-insulated 0.75 mm2
Mains-plug 3 pins 3 A fuse
Switch
  • DPDT.
  • On-On action.
  • 6.3 mm spade-terminals.
3 10 A
Wire Multi-strand Red & Black 16 AWG, 26-strand
Crimp-connector
  • Insulated.
  • Red or Blue.
  • Ring-terminal.
11 M3
  • Uninsulated.
  • F
  • Spade-terminal.
  • Female.
14 6.3 mm
Insulating boot For female crimp spade-terminal
Cable-tie Black 2 25 cm × 3 mm
Bolt Hex socket 2 M3 × 8 mm
Nut M3

From a glance at the list of parts, this is likely to be more expensive than the average commercial PVR, but then they're not exactly the same product. Whereas a commercial PVR will probably give you a nose-bleed when you discover that it promised long & delivering short, this solution is:

  • networked; one can connect over ssh, or HTTP.
  • flexible; there are many s/w add-ons to enhance functionality.
  • open-source; so you can in principle, inspect the code to see how your data is being used.
  • supported; the update-frequency depends on the selected distribution.
  • repairable.
  • many of the parts are re-usable in other projects, or may previously have been purchased for abandoned projects.
  • it looks better; OK, that's subjective, but I think it looks like Port Talbot at night.

If you're unconvinced by these merits, you could (in decreasing order of preference) reduce the cost by:

replacing the UPS with two wall-warts
  • reducing the power-on sequence to a process of inserting wall-warts sequentially into a distribution-board.
  • leaving the PVR vulnerable to brown-outs (which may be infrequent anyway).
  • the cost of a high quality power-supply for the Raspberry Pi (the USB-hub was presumably supplied with one) reduces the saving.
omitting the USB WiFi-adapter
requiring one to use the weak internal WiFi-adapter.
omitting the status-LEDs
requiring one to manually confirm that the HDDs & DVB-tuners are available after booting.
downgrading to just one HDD / SATA-USB adapter
increasing the chance that video-files will be lost on failure.
downgrading to merely one tuner (& also potentially remove the RF splitter & three gender-changers)
preventing simultaneous recording.
downgrading from DVB-T2 to the older DVB-T standard
limiting one to tuning into standard-definition broadcasts.
downgrading to a Raspberry Pi Zero W
  • only one CPU; slower but adequate.
  • only a mini HDMI port.
  • only one µUSB data-port.
removing the battery-backed real-time clock
the internet must be available when booting to permit synchronisation with an NTP-server, & prior to receiving this time-signal, all times will be incorrect.
PVR (plan) PVR (side elevation) PVR (side elevation) PVR (end elevation) PVR (end elevation)

Case

The case is built from acrylic.

  • This typically comes covered by protective plastic, which shouldn't be removed until completion. 3 mm was selected so that the grommets used to mount the HDDs could threaded through the width.
  • The A3 acrylic sheet is scored & snapped over a straight edge (which takes considerable force), into two A4 halves; one could merely buy A4 sheets (which I did for the bottom layer), but that's slightly more expensive. The sheets can then be clamped together to reduce the effort of subsequent processing & to force alignment of those holes they share.
  • The corners are hack-sawed & then ground to a radius of ≈1 cm; large washers can be clamped either side, to form a guide for the file. This is largely aesthetic, though my sheet of acrylic arrived by post with one corner pre-mashed.
  • Various holes were drilled using HSS twist-drill bits.
    • For accuracy a drill-stand was used, after piloting each hole by hand using a pin-vice & a 1 mm drill-bit (which easily bites into the relatively soft protective plastic).
    • Acrylic tends to ride up the thread of a twist drill-bit, & since it's brittle, the consequences are typically catastrophic, so each hole was progressively enlarged 1 mm at a time.
    • Excess heat can melt & discolour the acrylic, so a low drill speed was used & also peck-drilling for wider holes.
    • To reduce chipping, wood was placed beneath the acrylic to support the perimeter of the exit-hole as the drill-bit emerges from the far side.
  • A square-section acrylic bar is bonded to the lower side of the top acrylic sheet, between 2 holes from which both DVB-tuners are suspended using a cable-tie.

The horizontal acrylic sheets are connected to form two tiers, by vertical aluminium tubes at each corner, through each of which a threaded rod is inserted coaxially. Penny-washers are used to spread the load from the tightened threaded rods, to avoid cracking the acrylic. The length of the tubes separating the upper tier, plus twice the thickness of a penny-washer, should equal the 70 mm width of the HDD plus the thickness of the grommets. The length of the tubes separating the lower tier is less critical, but must exceed the combined height of the battery & crimp spade-sockets (the descending spade-terminals from the battery-switch above are offset from the battery). In either case a pipe-cutter is used to achieve perpendicular ends; using a hack-saw & file is considerably more laborious.

Rubber feet must be threaded onto the lower end of the threaded rods, since otherwise various protruding bolt-heads & cable-ties would prevent the case from sitting horizontally.

Computer

The Raspberry Pi's WiFi-antenna is very short (& there's no provision for an external antenna). Despite orientating the Raspberry Pi within the case so as to maximise exposure of the WiFi-antenna (located between the µSD-socket & the GPIO-pins), network-connectivity was insufficiently reliable, so it was replaced by a USB WiFi-adapter with a better antenna, & of sufficiently low power to permit direct connection to the Raspberry Pi's USB (rather than via the fully populated 4-port USB-hub). The integrated WiFi-adapter can then be disabled using the following commands.

echo 'blacklist brcmfmac' >/storage/.config/modprobe.d/blacklist.conf	# Disable RPi's weak internal WiFi.
echo 'blacklist brcmutil' >>/storage/.config/modprobe.d/blacklist.conf

The Raspberry Pi is attached above the middle acrylic sheet using 12 mm long standoffs to enable subsequent extraction of the µSD-card using one's fingers rather than tweezers. The upper end of the standoff must be female since there's insufficient space to tighten a nut; the lower end can be either gender.

There was apparently no need to attach a heat-sink to the Broadcom BCM2837 SoC. While recording two high-definition channels & watching a standard-definition recording, the temperature rose from a quiescent 39 C, to 57 C; well below the 80 C at which throttling occurs.

Though the Raspberry Pi has 4 USB-2.0 ports, it only has a single bus, limiting the data-rate to a theoretical 480 Mb/s. In the UK, each DVB-t2 tuner requires about 40 Mb/s. Though each HDD has a SATA 6 Gb/s interface, it won't be required to write any faster than both DVB-t2 tuners can receive. So if recording two HD channels while watching a third, the total requirement (200 Mb/s) is still within what might be expected from the USB-2.0 specification, provided that the Ethernet (implemented on USB) isn't being hammered for any unrelated reason.

The battery-backed real-time clock is connected to the GPIO-pins. Conveniently, the model selected passes the pins through for connection to other devices, but is only based on a DS1307 rather than the more accurate DS3231. To modify LibreELEC so that the kernel recognises it, ssh into the Raspberry Pi & issue the commands:

mount -o 'remount,rw' /flash;	# This file-system is normally mounted read-only.
echo 'dtoverlay=i2c-rtc,ds3231 # For realtime clock.' >>/flash/config.txt;	# Append a line to the config-file.
reboot;
hwclock -r	# Confirm the time.

Storage

The selected distribution is read from a µSD-card; I used a 32 GB (though 8 GB would have been sufficient) Samsung Pro Endurance, its read-speed easily exceeds the 25 MB/s speed of the card-reader (minimising boot-time), & claims significantly higher write-endurance. For installation, either follow "these instructions", or install NOOBS & select the required distribution.

Video files are recorded onto a RAID 1 built from a pair of HDDs formatted with BTRFS. To regularly maintain this filesystem, navigate down LibreELEC's TV-interface:

Settings → Add-onsInstall from respositoryLibreELEC Add-onsProgram add-onsBTRFS Toolsinstall
Settings → LibreELECServicescronEnable cron

Then ssh into the Raspberry Pi, & as the user "root" edit "$HOME/.cache/cron/crontabs/root" by issuing the command:

crontab -e	# Edit root's crontab.
0 2 1,16 * * $HOME/.kodi/addons/tools.btrfs-progs/bin/btrfs scrub start /var/media/Video

If this dumps you into an unfamiliar editor, then repeat after issuing the shell-commands:

echo 'export EDITOR=vi' >>~/.profile;	# Specify your favourite editor.
. ~/.profile	# Source the new profile.

These HDDs are connected to the USB-hub via SATA-USB adapters, & by rubber grommets to reduce transmission of vibrations & amplification by the case. The thickness of the acrylic from which the upper layers are composed, is limited to 3 mm to permit these grommets to be threaded all the way through.

Since kodi is not just a PVR, but a media-player, any audio files can be read from a USB flash drive. To avoid corruption of the file-system on power-loss (especially if you don't opt to build the UPS), ssh into the Raspberry Pi & issue the command:

echo "mount -o 'remount,ro' device" >>/storage/.config/autostart.sh	# Remount the file-system read-only.

RF

Two DVB-tuners were used, to permit simultaneous recording. I used a Geniatech MyGica T230 & a PCTV Systems tripleStick 292e, both of which worked seamlessly. DVB-T (Freeview in the UK) was piped to them via an RF-splitter. Initially I used a cheap 4-way splitter, which at best quarters the signal-power available to each output, resulting in a signal that was unacceptably weak, so I replaced it with a higher quality 2-way inductive splitter.

The assembly of RF-components was hung beneath the top acrylic sheet of the case, taking care to distance them from the GPIO-pins rising from the Raspberry Pi; connecting the 3.3 V pin to the 5 V pin, or shorting either to ground, is a great way to blow the Raspberry Pi.

USB-hub

The 7200 RPM Hitachi Z7K500s selected for storage of video files, require only ≈2 W each, but can draw a rather high 5.5 W on start-up (the 5400 RPM version is little better).

The power-requirements of both the HDDs & two 0.5 A DVB-tuners, exceed that which can be delivered from the USB-ports of the Raspberry Pi, so they're connected to a four-port powered USB-hub; I selected a hub which conforms to USB 3.0, not because of its speed (since the Raspberry Pi to which it is connected only supports USB 2.0), but because it makes 0.9 A available to each device, which is necessary for the HDDs.

I selected a 15 W Atolla 204u3. Input-power at 5 V is delivered via a barrel-connector, allowing both this & the Raspberry Pi to be powered using one buck-converter. The various USB-cables can be merely USB 2.0, though one might want to confirm that their power-wires are of adequate capacity; I just used USB 3.0 ones. The orientation of the USB-ports avoided the need to twist thick the SATA-USB cables through 90°.

Having allocated all of the ports of the USB-hub to high-power devices, I connected the USB flash-drive containing the audio files directly to the Raspberry Pi; this is tolerable, since its power-requirement is only 0.2 A. If one decided to replace the Raspberry Pi's WiFi with a higher-power adapter, then a six-port USB-hub would be a better choice.

Back-powering the Raspberry Pi from the USB-hub also, bypasses protection & is generally deprecated, so it retains its own independent power-supply.

UPS

The mains-plug has a 3 A fuse to account for the higher current on start-up.

The 230±10 V AC mains is converted to 12 V DC. Since the selected PSU is rated at 60 W but never exceeds 15 W, it only reaches 38 C, & since doesn't have any cooling holes beneath, it is mounted directly on the base acrylic sheet with two M3 hex socket bolts.

This is used to charge a sealed lead-acid battery, which returns the charge should the mains voltage be disconnected for any reason. Since the PVR typically takes ≈7.5 W, the 2.1 Ah 12 V battery can last for over 3 h. This battery is positioned to ensure that its terminals are in no danger of contact with the bolts for the HDD above.

This intermediate DC voltage is then stepped-down to supply both the Raspberry Pi & the USB-hub; both of which can tolerate 5±0.25 V. Since the selected buck-converter is rated at 10 A but only ever reaches a quarter of that, its heat-sink doesn't get very hot, but the supplied standoffs were sufficient to separate it from the acrylic base to increase air-flow. CAVEAT: I added insulating washers to these standoffs, which were otherwise uncomfortably close to circuit-traces. This module's voltage should be calibrated before use (which involves holding down a button before powering-on), after which it can be configured to display both i/o voltage & output current.

The buck-converter was set to the maximum tolerable 5.25 V. The measured voltage had dropped to 5.2 V @ the exit to the switch before the Raspberry Pi, at the maximum expected load, having lost a little over crimp-connectors, 16 AWG wires & one switch. Then there's the Raspberry Pi's µB USB power-connector which was constructed from a short fast-charge (no data) USB-cable, with one end removed & the two exposed 18 AWG power-wires (the thickest I could find) terminated with crimp spade-sockets.

The selected buck-converter also permits one to define a current-limit, & 3 A (calibrated by shorting the output into a 0.5 Ω 50 W resistor) was found to be adequate to avoid activation in normal circumstances. This prevents excessive loading (& consequently heating) of either the PSU or buck-converter, should a short-circuit occur (CAVEAT: it won't prevent disaster if a nut drops from the standoffs above, through the cooling-vents in the top of the PSU, since this is upstream from the buck-converter).

Switches have been used to independently isolate the 3 power-connections. This facilitates sequential application of power to the USB-hub & then to the Raspberry Pi, so that the HDDs are available to mount when the latter is booting. The battery is also connected via a switch, because removing the crimp-terminal (for whatever reason) is annoyingly hard. The minimum requirement is satisfied by SPST switches, but DPST switches allow one to simultaneously disconnect both power wires, & the greater functionality of DPDT switches (which cost little extra) may facilitate re-use in a different project. Each switch requires a 12 mm cut-out, which exceeds the diameter which can be achieved using a twist-drill bit without risking cracking the acrylic; I used a step drill-bit.

User-interface

The Raspberry Pi is connected to a TV using HDMI. The primary input-device is the TV's remote-control & infra-red receiver, which via HDMI-CEC can relay commands to the Raspberry Pi, avoiding any requirement for a dedicated infra-red receiver. CAVEAT: the HDMI-support on older TVs may not include CEC, or may need it to be enabled, & the manufacturer may refer to it using some fatuous brand-name (typically called "something-(link|sync)").

One can also connect over Ethernet, to:

Port Name Service
8080 Chorus kodi's web-UI
9981 Tvheadend One of kodi's possible back-ends for managing TV

Audio

Perhaps just when playing music, or also when watching a film, the output from the TV's internal speakers doesn't quite cut it. Your new TV may have a greater pixel-density than your eye can resolve, but the sound is probably no better than an old cathode-ray model; it's probably worse since the speakers typically face either backwards or downwards, to permit a stylish narrow front bezel.

One can connect an audio amplifier (& Hi-Fi speakers) to the TV's audio output (typically RCA sockets) rather than directly to the Raspberry Pi. The TV will probably have an item buried in its menu to redirect the audio signal. One could alternatively take the audio signal from the headphone jack on the Raspberry Pi.

Status-LEDs

Status-LEDs

The depicted circuit is powered from the Raspberry Pi's 5 V GPIO-pin, & can supply a constant current (irrespective of the LED's colour) to any of four LEDs, each according to the state of a dedicated GPIO-pin. Individual trimmer-potentiometers are available to fine-tune the brightness.

A daemon "show_status.py", governs the state of the connected GPIO-pins. To start this at boot-time, ssh into the Raspberry Pi & issue the shell-command:

echo 'nohup /storage/.config/show_status.py --poll --period=15 &' >>/storage/.config/autostart.sh	# Poll the h/w periodically.

This daemon could be used to continuously indicate arbitrary attributes of the PVR, but has been used to monitor the availability of the required h/w. CAVEAT: this script must be tailored to reference your specific hardware.

Both HDDs
References the file-system label.
USB flash-drive
References the file-system label.
PCTV Systems tripleStick 292e DVB-tuner
References the vendor & product.
Geniatech MyGica T230 DVB-tuner
References the vendor & product.

N.B. Failure to find some of this h/w might result from booting the whole system simultaneously. A more reliable procedure is to boot the USB-hub first (from which the above h/w is powered), & then the Raspberry Pi.


Conclusions

My impression is largely positive, but …

Operational Issues

  • The status-LEDs are neither self-explanatory nor particularly informative; a small LCD might be better.
  • The Python code above, polls for faults rather than reacting immediately.
  • If the whole system is powered-on simultaneously, then the hard-drives may not be visible when the Raspberry Pi boots.
  • Neither the integrated WiFi-adapter nor the replacement USB one, were satisfactory. The former suffered from an inadequate antenna & the latter drew too much current from the Raspberry Pi, resulting in sporadic "Under-voltage detected!" kernel-errors (despite not breaching the current-limit set on the UPS). A better (but more expensive) solution might be to use a secondary WiFi access-point to extend the range of one's network, compensating for the inadequacy of the integrated WiFi.

Physical Issues

  • The inflection of each SATA-USB adapter's cable, imparts an significant sideways force on the SATA-plug of the corresponding HDD, which may be need to be reseated after toggling the battery-switch (suspect this if the above fault occurs repeatedly). Fortunately the USB-plugs on the hub have the same vertical orientation as the SATA-plug, so that they don't also need to be twisted by 90°.
  • The top of each threaded rod needs to either be capped or sawn flush; I just haven't.
  • The aluminium tubes don't reliably centre around the threaded rod (as can be observed in the photos), which was partially resolved by inserting M3 × 9 mm washers into the base of each.
  • The snapped edge of the acrylic isn't as precise as hoped; perhaps it can be smoothed with a blow-torch, or one could just buy A4 sheets in the first place.
  • The battery-switch is inconveniently located, & clumsy use may unseat the SATA-plugs.