in the conventional system shown in fig1 , a spread tow 2 of plasticised cellulose acetate filaments , which has been subjected to the usual pre - treatment stages ( not shown ), is gathered towards rod shape by funnels 27 , 28 as it advances to plugmaker 55 , which forms it continuously into elongate filter rod 57 . plugwrap 52 from a supply roll 50 , and the tow 2 , are conveyed through the plugmaker 55 on and by a conveyor 54 which also wraps plugwrap 52 around the rod as the rod is formed and secures it in place by means of a lapped and stuck seam . rod 57 passes from conveyor 54 via rolls 58 , 59 to a cutting device 60 which severs the formed rod into finite lengths 61 . the gathering or condensing means 27 , 28 of fig1 could be replaced by a single gathering funnel or the like . such a single gathering funnel 4 is shown in fig2 , where 2 is the tow supply as in fig1 but the plugmaker etc . of fig1 is omitted for clarity . in fig2 carbon granules 6 from a supply reservoir 8 are discontinuously injected radially into the gathering tow in funnel 4 through injector barrel 10 by means of an injection mechanism 12 shown in more detail in fig3 . the carbon granules are conveyed pneumatically along injector barrel 10 and exit the barrel to form pockets 14 embedded in and spaced along the continuously produced filter rod 57 ; whilst pockets 14 are shown in fig2 , they would of course not be visible in the rod in practice . the carbon supply 16 to reservoir 8 is maintained under pneumatic feeding pressure from main tank 18 . air pulse generator 74 , controlled by electric motor 34 , receives high pressure air from compressor 22 and directs rapidly repeating high pressure air pulses into injection mechanism 12 at 24 to correspondingly repeatedly re - open a valve of mechanism 12 , the valve being closed between said pressure pulses by constant push - back air pressure from 26 . in operation , the valve thus oscillates to repeatedly shut and re - open very rapidly . as the valve opens momentarily at 46 and until it closes shortly thereafter , carbon granules enter barrel 10 from reservoir 8 ; entering particles are immediately separately carried rapidly along barrel 10 and injected radially into the gathering tow by a high velocity flow ( e . g . 100 to 200 or more meters / second ) of driving or conveying air which is passed continuously into barrel 10 from 20 , and virtually instantaneous conveyance and injection of entering granules continues until the valve closes to momentarily stop the granule feed ; carbon granules are thus discontinuously injected radially into the passing tow to form spaced additive pockets 14 in the product filter rod ; the tow throughput and the speed and timing of pneumatic injection are such that the tow advances only a short distance during each injection , facilitating formation of a product rod with well - defined spaced granule pockets . the stroke , or opening travel , of the valve of injection mechanism 12 is limited by a stop 28 whose position is determined by cam 30 adjustable by an electric motor 32 controlled by flow rate controller 76 . a cutting device 36 severs the continually produced rod 57 to finite lengths such as those shown at 61 , these usually being an even multiple of ( e . g . 2 or 4 or 6 times ) the length of the eventual individual filters . the cutting device 36 , by way of infrared registration cell 38 , encoder 40 and controller 42 with user interface 44 , is synchronised with the tow feed and controls synchronised operation of the injection mechanism to ensure cutting only between the embedded pockets and not through a pocket . if conveying air from 20 enters funnel 4 it may be vented from the filtering material before the latter is fully shaped to rod form , e . g . via apertures ( not shown ) through the wall of funnel 4 . additionally or instead there may be venting or extraction of conveyor gas laterally out of barrel 10 between valve opening 46 and the granule injection outlet . thus arrow 19 indicates such optional gas venting or extraction outside of the gathering filtering material and funnel 4 ; this could be by way of an outlet port or ports ( not shown ) through the wall of conduit 10 , or through piping ( not shown ) connecting the interior of conduit 10 to a vacuum source ; in the latter case the volumetric vacuum outflow rate may be high enough ( e . g . greater than the volumetric inflow rate from 20 ) to remove unwanted dust and carbon fines but without unduly affecting injection of the larger granules for pocket formation . the injection device 12 of fig2 is shown more clearly in fig3 ( a ) and 3 ( b ) in which its valve 13 , 48 is shown respectively open and closed at 46 . fig3 ( a ) shows carbon granules entering injector barrel 10 through the opening at 46 ( see also fig2 ) of valve 13 , 48 within the reservoir 8 . a high pressure air pulse at 24 is shown acting on piston 48 of valve 13 to push it back into the air - spring chamber 70 against the push - back pressure from 26 , momentarily opening the valve at 46 , to the extent permitted by stop 28 , to allow the entry of carbon granules into injector barrel 10 . fig3 ( a ) indicates granules 6 dispersed into a relatively diffuse stream by their rapid pneumatic conveyance away from the valve inlet 46 . on cessation of the high pressure air pulse at 24 , then as shown in fig3 ( b ), the push - back pressure from 26 recloses the valve with exhaust air venting at 72 and with the carbon granules having been carried away and injected radially into the gathering tow through barrel 10 by the constant supply of driving air from 20 . fig3 ( b ) indicates the final few granules 6 which entered conduit 10 immediately before full closure of the valve at 46 . it is emphasised that the representation of granules 6 in conduit 10 of fig3 ( a ) and ( b ) is purely schematic . the position of adjustable stop 28 determines the maximum size of inlet 46 of the valve ; for given operating conditions ( reservoir pressure , valve movement speed , and time for which the valve is fully open ) product pocket size is thus simply adjusted by adjustment of stop 28 . in the embodiment and modifications thereof described above with reference to the drawings , the injector barrel 10 extends radially of the axis of the filtering material path , but it could instead be non - perpendicular to the axis — e . g . extending obliquely through the open upstream mouth of the gathering device to within the gathering tow . different patterns of embedded additive pockets in the product rod can be obtained by adjustment of the pattern of air pulses at 24 and hence of the pattern of opening and closing of the valve of the injection mechanism . fig4 illustrates three possibilities for additive pocket location in filter rods according to the present invention . the illustrated quadruple length rods supplied for filter cigarette manufacture would normally be severed first along line b to give two double length rods ; each double length rod would then have two tobacco rods attached thereto , one at each end , followed by cutting along line a to yield two filter cigarettes . in option ( a ) the fully enclosed pockets 14 are equally and uniformly spaced along the rod , and in the eventual individual filter on a filter cigarette the pocket 14 would be centrally located . in option ( b ), the valve of the injection mechanism is operated to give alternating close and wide spacing of succeeding pockets 14 , and the initial cutting of the multiple length rod from the continuously produced product is such that , in the filter cigarette product made as described above , the additive pocket of the individual filter is displaced towards the buccal end . preferred is option ( c ), where the continuously produced rod has the same pocket pattern as for ( b ), but the initial cutting to give the multiple length rod is such that the eventual individual filter has the particulate additive pocket 14 displaced towards the tobacco end and remote from the buccal end ; this reduces or eliminates risk of carbon marring the appearance or taste of the filter cigarette . preferred filter rods of the invention , as illustrated , have the filter material matrix free of stray injected particles , and the matrix and additive pockets substantially free of dust and additive fines . the representation of the additive pockets in fig4 is diagrammatic ; in practice each pocket preferably has a more curved surface , being generally ellipsoidal or rugby ball - shaped . the method and apparatus according to the invention can produce composite additive — carrying filters of conventional size , carbon content and performance . the individual product filters may for example be of conventional circumference ( e . g . about 25 mm ) and length ( e . g . down to 27 or 25 mm long ) and have a conventional carbon content of about 15 to 35 mg — or an even higher carbon content of up to 60 mg ; for longer tips , higher carbon content is possible . the filters have a filtering performance similar to that for conventional dual filters of the same carbon content . each particulate additive pocket , in a rod of 25 to 32 mm length , may for example be from 10 to 18 mm long with a diameter of 3 to 4 mm which may reduce somewhat towards each end . the continuous single - pass method and apparatus of the invention can be operated efficiently at commercial speed ( e . g . over 200 m per min ); transverse , e . g . radial , pneumatic conveyance and injection of the particulate additive maintains separation and maximises accurate location and confinement of the pockets thus reducing or eliminating rejects or variable quality product due to additive dispersal or to pocket coalescence ; this is because the transverse pneumatic travel path can be short — for example , in the illustrated device the distance from valve inlet 46 to the point of injection may be only about 135 mm ., and even shorter distances are feasible . the pneumatic injection device employed in the present method and apparatus is advantageous in itself , being compact and efficient and readily fittable to most or all conventional cigarette filter making machines . thus such fitting to conventional machinery requires at most minor modification or replacement of the gathering funnel to accommodate a lateral injector barrel or conduit , and / or perhaps to provide additional vents for exhaust of pneumatic injection gas ; and even such minor modifications may not be needed if the injector barrel is to extend obliquely or axially of and through the open mouth of the gathering device and / or there is provision for lateral extraction of conveyor gas upstream of the particle outlet of the injector barrel and outside of the gathering device . accordingly , the invention also provides a device for use in injecting particulate additive into a train of tobacco smoke filtering material , the device comprising an injector conduit mountable to extend into ( and preferably transversely of ) such train and having a valve for discontinuous supply of particulate additive to the conduit , means for repeatedly opening and closing the valve so that particulate additive can enter the conduit when the valve is open , and means for receiving a constant high velocity stream of conveyor gas into the injector conduit to convey said supplied particulate additive along the conduit for discontinuous pneumatic injection into such train . the valve is preferably the same as or similar to that illustrated in fig2 and 3 , as is the means for oscillating it between open and closed positions . the additive supply is preferably from a reservoir for receiving and holding particulate additive under pneumatic pressure , and more preferably the injector conduit extends through the reservoir . the device can have , upstream of the particle outlet of the conduit , means for venting or extracting conveyor gas as described above and for the purposes indicated above .