Window shade having balancing drive system

A window shade, comprising:

• a roller, a first bracket and a second bracket, wherein the first bracket and the second bracket are disposed immovable relative to an architecture and spaced apart from each other;
• the roller is disposed between the first bracket and the second bracket, and is operable to rotatable around a first axis;
• an extendable component, having one end edge connected to the roller so that the extendable component is configured to be wound onto or unwound from the roller;
• and
• a balancing drive system, operatively connected to the roller, comprising:
• a base, disposed to correspond to one end of the roller and disposed within a chamber of the roller;
• a power assembly, disposed on the base, comprising:
• a rotary-drive member, rotatably connected to the base;
• and
• a coil spring, having one end connected to the rotary-drive member so that the coil spring is configured to be wound onto or unwound from the rotary-drive member according to a rotation direction of the rotary-drive member;
• an adjusting assembly, comprising:
• an output reel, connected to the base, rotatable around an output-center axis relative to the base, and drivingly connected to the rotary-drive member so that the output reel is subjected to an elastic torque substantially continuously provided by the coil spring;
• a receiving reel, connected to the base and having a receiving-center axis parallel to the first axis, wherein the base and the receiving reel are configured to allow for a relative rotation therebetween;
• and
• a transmission cord, having a first end and a second end fixed on the output reel and the receiving reel respectively, wherein the transmission cord is wound on at least one of the output reel and the receiving reel, and remains in a taut state;
• and
• a friction member, directly or indirectly connected with the power assembly, and providing a frictional resistance to the roller;
• wherein when the extendable component is operated by an external force to move, the extendable component is wound onto or unwound from the roller, wherein when the extendable component is unwound from the roller, the transmission cord is wound successively on the receiving reel to form plural receiving loops;
• when the extendable component is wound onto the roller, the transmission cord is wound successively on the output reel to form plural output loops;
• the plural receiving loops and/or the plural output loops are wound with radii thereof gradually varying correspondingly with a released length of the extendable component unwound from the roller.